Your goal is to grow a sapling out of the darkness and into the light where it can flower. If you manage to get enough flowers on it you complete the level.
The way you grow the sapling is the crux of the game. You have to prune branches that aren't going the way you want them too. The plant grows organically and you're only control over it is pruning the plant.
It's a simple mechanic but it's an amazing amount of fun. This has been my go-to phone game lately and I would encourage all of you to check it out.