XP is also known as EXP, and it is a short abbreviation for Experience Points. XP is a very useful and important thing in the game, it will helps you level up. All this experience can be earned from training items and completing tasks. Experience Points can be earned very fast and as you level up you have to earn even more experience points to earn or less to earn. There are many simple ways to earn experience points and sometimes many hard ways. However, experience points can be earned for almost anything you do. For example, tickling your ninja can sometimes earns XP. But the probably most common is to throw your ninja around and if he goes on his feet you earn XP!
The main way to earn XP is to train your ninja, as well as completing some assignments that the squirrel, chicken, sensei, or the ninja assigns you.