Experience, or XP, is a useful Minecraft mechanic. XP orbs look like a blue and yellow sparkling ball, and naturally try to get to the nearest player or NPC.
XP is dropped by mobs, and users upon death. Mining, smelting, trading, and breeding also gives you XP.
Experience can be used to enchant, and server-jump, or go to the next server on the server pyramid. This is what allowed Gameknight to survive the TNT blast in Invasion of the Overworld.