Wherein shall be Explained the [Teaching] of the Gemara [Which Discusses] What the Profession of a Person is in this World…
We have learned in [maseches] Chullin (89a): “Rabbi Yitzchak said, ‘What is the meaning of that which is written [in Tehillim], “The trade is silence, they speak [words of] righteousness, they judge people with justice.” (Tehillim: 58; 2) What is a person’s trade in this world? He should act like a mute. The pasuk terms [the attribute of silence] as a “trade”, in order to teach us a number of different matters: [The first thing that can be learned from the pasuk in Tehillim is that] it is known that a person who is not a craftsman who wishes to make a given vessel, even though in his mind he illustrates the procedure in which to make [the vessel], in all of its facets, with ease, it will nevertheless be difficult for him to make [that vessel] in actuality, since his hands are still unaccustomed to [producing the vessel. The above] is not true of a craftsman who is accustomed with this [type of work] from his youth.
[The above discussion of the craftsman] is [similarly] true regarding the attribute of silence: All of those who [are capable of] comprehension, view this attribute [of silence] to be exceedingly good, for [with silence one] is guarded from [transgressing] all of the prohibitions that come through speech, and, [on the converse], without [this attribute of silence], he will likely [arrive at] a number of situations [where he will speak that which is forbidden], as shall be explained below. However, if a person will consciously agree to only grasp this attribute [of silence] at a time when it is [absolutely] necessary, due to a mitzvah of the Torah [which obligates him to be silent], such as refraining from speaking Lashon HaRa, Rechilus, [words of] mockery, and the other types of forbidden speech, and with the exception of [forbidden speech], he will speak all that he wishes, [including] superfluous verbiage, [in this case], he will definitely not abate from [these types of forbidden speech] as well. On the contrary, [since] from his youth he has accustomed [his mouth and tongue] to speak of everything which he thought, [he is more prone to speak that which is forbidden. The above] is not true of a person who has accustomed his mouth with the attribute of silence to the extent that a craftsman is accustomed with his craft, to the point that the [attribute of] silence will become something that is of [second] nature to him, and speaking will be outside his nature. [This person, whereupon making the attribute of silence part of his nature, to the extent that he is] similar to a person who is a mute, his heart is definitely prepared to be confident in HaShem, that he will guard his tongue from evil, and never again come [to speak] words of foolishness.
 The translation above is based on the gemara in maseches Chullin.
The following is the translation that is based on Onkelos:
“Is it really true that the righteous are silent? At the time that they are found, you observe that they speak [words of] righteousness, they judge people with justice.”
This verse begins with the word "האמנם", which is generally translated as “Is it true…?” This word is explained in the gemara in Chullin to refer to "אומנות" – “a trade”. The gemara explains, based on this pasuk from Tehillim, that a person should act as a mute, refraining from speaking, with the exception to words of Torah. However, they should not think that they can become haughty, for they must “judge people with uprightness.
RaSh”I explains this pasuk in the context of David taking the spear from Shaul, while Shaul and his men were asleep. This incident occurred when Shaul was in pursuit of David. David called out to Avner, the general of Shaul, informing him that he could have killed Shaul, had he wished. In this chapter of Tehillim, David comments on this incident, “Have the words of righteousness that you should have spoken [about me], and the uprightness with which you should have judged… been silenced from your mouths?”
According to the RaDa”K, Shaul’s men could have prevented Shaul from pursuing David. However, they justified Shaul’s pursuit of David, claiming that David was rebelling against Shaul’s kingship.