XP stands for "Experience Points". You will earn XP each time you play Bingo - earning XP allows you to level up, and reaching certain levels will unlock new rooms to play in!
Regular Bingo Rooms are unlocked by reaching a certain player level - you can see the level you'll need to reach on each "locked" Room icon. Your current player level is the number beneath your profile picture in the top left of the Room Selection screen.
Your current XP progress until the next level is tracked from the border of your profile picture. The border will start to fill with a green colour when earning XP. Once the border has been completely filled, you will advance to the next level.