“This scroll of the Law must never cease being spoken out of your mouth.” In this sentence, the “scroll of the Law” is put by metonymy for the words that are on the scroll of the Law.
“meditates.” The translation “meditates” here in Joshua 1:8 is okay if it is understood, but it is not a perfect match with the meaning of the Hebrew word. If it is not properly understood in the biblical culture and context, it can lead to misunderstanding. The Hebrew word is hagah (#01897 הָגָה), and when used of humans its basic meaning is to utter a sound. Thus, it can mean to mutter, moan, utter, speak. It can mean to read out loud in an undertone. Its extended or applied meanings can include to recite, muse, imagine. In any case, what it does not mean is to think about in silence, like the silent monks. God wants us to read, recite, think about, and dwell on His Word and works, especially out loud. The idea is to memorize it, if not word for word, to certainly get to the point we know what God’s Torah says and means. The HCSB may get the sense of the Hebrew better than “meditate” when it says, “you are to recite it day and night.” The NET paraphrases the meaning when it says, “You must memorize it.” The Torah is not a “meditation” in the yoga sense of the word. We are to repeat it over and over, including out loud, until we know it. If Joshua was going to be truly successful, not only as a warrior but as the one who, after conquering the Promised Land would establish the foundation of a godly society, he had to know the Torah, God’s “instruction book” and guide to godly thinking.
Both Joshua 1:7 and 1:8 make the important point that if we want to be successful and prosper in this life—from God’s point of view, not necessarily the world’s point of view—we must know and act on the Word of God.