When you finish a run, you have the option to tap and view a video ad. Your reward if you do so is 20 gems.

You can purchase a micro-transaction to eliminate the ads and just enjoy the game.

If you don't want to pay for those ads just switch your game to airplane mode. However keep in mind video viewing will be disabled as well.

Aim your jump correctly ie just when your character drops very slightly tap the screen to jump.