It is written [in Mishlei : 21, 23]: “One who guards his mouth and tongue, guards [himself] from troubles of his soul.” To understand the concept behind why the pasuk specifies guarding one’s speech as a preventative measure against the troubles of the soul (more so than all other matters that the Complete Person must guard during his lifetime), refer to those commentators who explain the simple meaning of the pasuk. I will provide my own answers (with which HaShem has graced me), through which we shall also understand that which is written [Tehillim (34; 13-14)]: “Who is the man who desires life, who loves days to see good? Guard your tongue from evil….”
It appears that the reason the pasuk has specifically pointed out avoidance of Lashon HaRa, is because it is known that every man has 248 limbs and 365 ligaments which are spiritual. [These spiritual limbs and ligaments] are clothed in the 248 limbs and 365 ligaments that are physical. [The above follows that which] it says [in sefer "Iyov"]: “[With] skin and flesh You have enclothed me, and with bones and ligaments You have covered me.” (Iyov: 10; 11)
Behold, the pasuk mentions skin and flesh and ligaments and bones, and it describes them as “clothing” and “covering”, as it says in the pasuk “talbisheni” – “enclothe me”, “tisochicheni – cover me”. Whom did [HaShem] enclothe, if not the soul that resides in the body which is the essence of the person? Every single spiritual limb is enclothed in a physical limb that mirrors the will of that spiritual limb, [this being] analogous to clothing [covering] the body. In relation to this, The Holy One, Blessed is He, gave us 248 positive commandments and 365 negative commandments that are associated with the limbs. There is a commandment that is dependent upon the hand, and there is a commandment that is dependent upon the foot, similarly [such is the case with] all of the other limbs, as it says in Sefer Chareidim (that the mitzvos are enumerated according to the number of limbs of the person). When a person fulfills any mitzvah with any limb in this world, [then] the light of HaShem will rest upon that limb in The World to Come . It is that [spiritual] light that provides life to that limb. This [is the case that the aforementioned spiritual, life-giving light rests on each of the limbs, as a result of the performance] of every mitzvah. We see that when a person fulfills 248 positive commandments, he becomes the complete person that is holy to HaShem with all of his limbs. [The aforementioned] is what is referred to in the chapter of tzitzis, [when it states] (Bamidbar: 15; 40), “...and you shall perform all of My commandments and you shall be holy to your G-d.” However, if Heaven Forbid, one will be lacking one mitzvah from the 248 positive commandments, that he neglected and for which he did not repent, [then], in The World to Come, he will be lacking the spiritual limb that corresponds to that given mitzvah. [The above teaching relates to] that [which] is what is [taught] in [maseches] Berachos (26a): “That which is perverted cannot be repaired” (Koheles: 1; 15) – This [pasuk] refers to one who neglected the recitation of Shema of the evening or the recitation of Shema of the morning, or the prayer of Ma’ariv or of Shacharis. [In contrast], when a person is careful to refrain from transgressing the negative precepts of the Torah, he draws the Holy Light onto the ligaments of his soul; and when he is not careful [to refrain from transgressing the negative precepts], Heaven Forbid, [those ligaments] will be crippled [in The World to Come]. This [teaching] is explained at length in the sefer “Sha’arei Kedushah” in the first chapter.
 “Pasuk” refers to “verse”.
 “HaShem” (literally “The Name”) is one of the references to G-d.
 “Tehillim” is the book of Psalms.
 The full quote is: “Who is the man who desires life, who loves days to see good? Guard your tongue from evil and your lips from speaking deceit.” (Tehillim: 34; 13 - 14)
 “Lashon HaRa” literally means "evil speech" and refers to slander, such as when one relates information about their fellow in a manner which may cause harm or degradation to befall that individual as a result.
 Any time the pronouns such as “He”, “Him”, “My”, or “You” are capitalized, they refer to HaShem. In this pasuk, “You” refers to “HaShem”.
 “Iyov” refers to the book of Job.
 The literal translation of this verse would be “Whom did He enclothe...” instead of “Whom did [HaShem] enclothe...”.
 “Mitzvah” is a biblical command, which is either positive (i.e. “do this”) or negative (i.e. “refrain”).
 Tzitzis – Literally, “fringe”. Refers to a four-cornered garment with fringes on each corner, discussed in (Bamidbar: 15; 37-41).
 “L'Asid Lavo” - literally, “In future times” can have varying meaning, depending on the context. In this context, “L'Asid Lavo” refers to The World to Come, where one will partake of their eternal reward. In other contexts, “L'Asid Lavo” can refer to the Messianic era, or to the post-Messianic era.
 “Bamidbar” – “Numbers” – fourth book of the Pentateuch.
 Berachos – The first masechta (tractate) in the Talmud.
 The entire pasuk reads, “That which is perverted cannot be made straight, and that which is lacking cannot be counted.” (Koheles: 1; 15) – "מעות לא-יוכל לתקן וחסרון לא-יוכל להמנות:" (קהלת: א, טו) RaSh"I (1040 - 1105), one of the foremost Jewish commentators on the Written and Oral Torah, explains, “That which is perverted in his lifetime, cannot be fixed upon death… and our Rabbis have explained that one who has a forbidden sexual relation and thereby fathers a mamzeir (child born from an adulterous or incestuous relationship), or a Talmid Chacham (Torah scholar) who leaves Torah, are cases when something was originally straight but had become perverted.”
 Koheles – The book of Ecclesiastes, written by King Solomon and read on the holiday of Sukkos (Tabernacles).
 The “Shema” includes portions from the Torah that deal with acceptance of the yoke of Heaven upon oneself, the consequences of following the mitzvos or refraining from their observance, and the portion that deals with the mitzvah of tzitzis and the Exodus from Egypt. The Shema is recited twice daily, in the morning and evening prayers. The Shema, or a portion thereof, is also recited upon going to sleep.
 “Ma'ariv” is the evening prayer.
 “Shacharis” is the morning prayer.
 “Sefer” is a book.
 Sha’arei Kedushah is a book, which means “Gates of Holiness”.
 This teaching is found in the first section and first gate of sefer “Sha’arei Kedushah”.