XP stands for Experience Points. These are collected for each question you answer correctly, regardless of the final outcome.
When you collect enough XP you will gain a level, or level up. All Cities have level restrictions; if you're XP level is not a high enough, you can't play in that city yet.
You can check to see how many XP you have and how many more you need to level up by tapping your profile picture in the upper left corner of Know It or Blow It home screen.