This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Regardless of what recursion is, surely you can run and debug (if necessary) to see what this code is doing. Once you have done that and researched recursion using our friend Google, come back and ask about what you don't understand.