Experience Points, or XP as they are sometimes called in shorthand, are a measurement of a character or item's growth in a video game.
The more you use certain characters, weapons or even vehicles in-game, the more Experience Points those items will get and the more powerful they will become and the more skills they will unlock.
What, exactly, is earning experience depends on the game. In Call Of Duty: Black Ops 2, for example, both characters and guns earn Experience Points, while in many RPGs, such as The Elder Scrolls V: Skyrim, only the characters earn Experience Points.
