I'll attempt to explain, but only with this caveat: You really have to try it to understand the appeal. Also note: The game gradually rolls out mechanics and explains them well.
So you're a space beetle hurtling along a silver track. A number of course elements will come your way. I will give some examples, but it's not comprehensive:
- Glowing panels. When these pass by you, hit A to "thump" them. You can safely pass over them without thumping; however, you score points by thumping. And, during boss battles, you have to hit all of them in a row and then hit the boss's charge shot (a bright green light coming down the track) to repel the shot back down the track at the boss.
- Orange lines of light. These are horizontal on the track. As these come down the track, you need to hold A to break through. There are often many in a row, with panels in between, so you'll have to alternate between tapping and holding. If you hit one without holding, you'll take damage.
- Spike traps. Resemble pink spikes. You need to tap forward and hold A to fly over them. If you hit them, you'll take damage. You can also fly off glowing panels to reach optional rings over the track. Also, when there's a panel at the end, you can pull back on the stick to SLAM down. This will expel a beam of light that will break a boss's shield, at which point you can hit them with a counterattack. Can also be used on glowing panels for more points.
- Walls. These pop up at corners and have red flashing lights. Some are long, some are short. You hold in the direction you wish to turn while holding A during the turn. You can start holding A and turning early, before you reach the wall, but it appears that for the most points, you need to do it right upon reaching the wall. If you hit a wall, you'll take damage.
There are more mechanics beyond these. In World 4 they start adding multiple rails you may find yourself shifting between five of them at a time, dodging incoming enemies such as silver centipedes that come hurtling down the track.
If you take damage once, your beetle loses its chrome armor. Take damage again, and it's back to the beginning of the stage. You can get back your chrome armor by thumping certain blue panels, such as the ones that always appear at the end of each stage.
It doesn't sound like much on paper. But in practice, it's incredible.