Celeste has three core moves: Jumping, climbing walls, and dashing. But it's also a game about a lot more than that. It's about overcoming obstacles.
It's about overcoming the physical obstacles of each level as you climb the mountain, sure. And, tangled up in that, it's about Madeline overcoming her personal anxieties to focus on the task at hand. As you get higher and higher, you learn more about her abilities at the same time as learning about her mental struggles.
Without the story, it's still an exceptionally strong platformer, and has become popular with speedrunners. Speedrunning as a general concept sounds, y'know, pretty simple. But then you actually watch how the Celeste speed-runners play the game. It's beyond comprehension.
Even the helpful speed-runners, who have mapped their button inputs so that they show up on the screen, are still doing so many things at any given time that they move at a bewildering pace. This world record run at the time of writing is fascinating to watch, and the record holder, who goes by the YouTube handle of "TGH", says that it should have been around 27 minutes and 20 seconds. Shaving off that time is his mountain to climb now, and he's already at the peak of having a world record run. So whatever your goal with Celeste, it seems like it's worth climbing.