Experience points (XP) are gained after every spin, depending on how many lines and coins you are betting per spin. The higher your bet is and the bigger the win, the more experience points you will gain towards advancing to the next level.
You also get a bonus every time you level up! Remember, as you increase in levels, more experience points will be required to advance to the next level. The great news is this will increase your bonuses per level up as well!