Joshua 1:8
8 This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success.
The book of the law refers to the first five books of Moses, Genesis, Exodus, Leviticus and Deuteronomy.
To mediate on this book, is to keep in your heart, that which God has instructed. By doing as God has instructed your life will be blessed and you will prosper. Good success refers to no strings attached to that success.
The larger context of this verse is that Moses had died and God was telling Joshua that if he followed the same edicts God had given Moses, that Joshua and Israel as a whole would prosper.
Joshua 1:1–9