The mechanics are very simple: you have to tap the screen so that the Santa flaps, trying to keep a steady rhythm in order to pass through the wall scattered through its path. It won't be at all easy to do.
It will be a challenge to get more than five points (which means flying through five walls), as it's really difficult to maintain a steady flapping rhythm. In fact, you will have to practice a lot before you start doing well and the game stops being so frustrating.
Each time you crash and start over, you will notice slight differences that make the game more interesting. You will play in different scenes, at different times.
Flappy Santa is an arcade game with very simple mechanics that presents a very difficult challenge. This is precisely why it's so successful, because it sucks you in with just the right amount of difficulty and makes you want to keep trying it one more time.