XP, or Experience Points, are the points you gain with every spin to increase your Player Level. As you level up, more Experience Points are required to reach the next level. You'll gain early levels quickly but as you get higher up in level, the longer it will take. The amount of XP earned is based on your bet per spin so raising your bet will increase the amount of XP earned for each spin.
As your player level increases, you will be able to unlock various bonuses and features in the game!