At the top of your screen, there is a number next to a blue bar. The number is your current level, and the bar shows how far you have to go before reaching the next level.
Every time you restock a shop or finish building/upgrading a building, blue stars will make their way to that bar to fill it up. The number of stars you get per action depends upon the shop you restocked or what you built/upgraded. The number of stars it takes to fill up the bar increases with each level.
You will level up much faster (and earn many more coins) if you keep the game open and restock stores whenever it's needed.
Every time you gain a level, you earn 1 Flooz. There are also shops, houses, and decorations to be unlocked at most levels up to level 45.
Currently, the level cap is 100, but it gets harder and harder to progress after level 45.
XP: Experience points required for the next level.