The entire pasuk states, “This Book of The Torah shall not be removed from your mouth and you shall contemplate its [teachings] day and night, in order that you shall guard [yourselves] to perform in accordance with all that which is written in it, for then you shall be successful in your paths, and then you shall be wise.” (Yihoshua: 1; 8)
Metzudas Tzion explains that the word “תהגה” could either mean “utter”, as in one should utter words of Torah day and night, or it could mean “contemplate”, in that the person should contemplate words of Torah during the day and night. The RaDa”K also offers the same explanation.
 The text from “Sifrei” states as follows:
“Rabbi Yishmael used to say, ‘“Only guard yourself and guard your soul exceedingly…” (Divarim: 4; 9) [This is] comparable to a king of flesh and blood who trapped a bird and put it in the possession of his servant. [Thereupon, the king] said to [his servant], ‘Be careful with this bird, [for it is] for my son. If you have lost it, don’t think that you have lost a bird valued at an isar, rather, it is as if you have lost your life. Similarly, it says [in reference to The Torah], “For it is not an empty matter from you…” (Divarim: 32; 47), [that which] you say is empty [when referring to The Torah] is that which is your life.””
 In reference to The Torah, this pasuk states, “Only guard yourself and guard your life exceedingly, lest you forget the things which your eyes have seen [i.e. at Har Sinai – Commentary on Yonasan ben Uziel] and lest you shall remove them [i.e. these words of The Torah] from your heart all of the days of your life, and you shall teach them to your children and to your children’s children.” (Divarim: 4; 9) The next pasuk states, “The day [on] which you stood before HaShem your G-d at Choreiv (Har Sinai), when HaShem said to me (i.e. to Moshe), gather the nation for Me, and I shall make My words heard [by] them, [so] that they shall learn to fear Me all of the days that they are alive on the ground, and [so] that they shall teach it to their children.” (4; 10)
 This pasuk, also in reference to The Torah, states, “For it is not an empty matter from you, for it is your life, and in this matter you shall lengthen your days on The Land which you are crossing the Jordan [River to go over] there [in order] to inherit it.” (Divarim: 32; 47)