What did God tell Joshua to meditate on day and night?

Answer: The Book of the Law

The Book of the Law was not to depart from Joshua's mouth; he was to meditate on it day and night and do all written in it.

“This Book of the Law must not depart from your mouth; meditate on it day and night, so that you may be careful to do everything written in it. For then you will prosper and succeed in all you do.”— Joshua 1:8 (BSB)