This book of the law shall not depart out of your mouth, but you shall meditate thereon day and night, that you may observe to do according to all that is written therein: for then you shall make your way prosperous, and then you shall have good success.
Now God is saying, "Look keep the law, don't deviate from it, for it is by this observing it day and night, meditating in it, thus thou shalt make thy way prosperous, thus you will have good success."