XP is needed to progress to the next level in It Girl & unlock the stores & items that come with that level. XP can be collected by shopping, showdowning with other girls, winning parties, etc. Check the guide below for the full list of available activities.
All new players start from Level 1. The first few levels are the easiest ones to beat. The more you progress, the more xp you require. Once you reach the next level, you "level up".
At the moment It Girl has 145 possible levels. It is quite possible that more will be added in the future.
Also, if you are at level 139 or close enough, you might be wondering where to find a lot of XP to level up during levels 139 - 145. To level up fast in a week per level or even less, check the following guide.