Joshua 1:8

This book of the law shall not depart out of thy mouth
He was often to read it, frequently repeat it, and speak of it, to refresh his own memory with it, and the memory of those about him. Jarchi restrains it to the book of Deuteronomy now before him:

but thou shalt meditate therein day and night;
whenever he had any leisure from the important business of his office, whether by day or night, see ( Psalms 1:2 ) ;

that thou mayest observe to do according to all that is written
therein;
which frequent speaking of it, and constant meditation on it, would lead unto:

for then thou shalt make thy way prosperous, and then thou shalt have
good success;
in his wars with the Canaanites.