Experience Points, also known as Xp, is an item you need to score in order to level up your town. Most of the in-game actions give you Xp.

You get experience points for harvesting crops, building houses, community buildings, zoo enclosures, factories and special buildings, completing land expansions, buying and selling decorations, filling orders at the helicopter pad and in the Zoo, helping friends and co-op members, completing achievements and other in-game actions.