Some allow you to enable better graphics than the native version, some can downgrade graphics to improve performance. Some remove framerate limits. Some are cheats, with infinite health and similar. It varies a lot, really.
For example, with Tears of the Kingdom on a Steam Deck there are mods to make the game run at 16:10, with better performance, and better frame pacing at 30 FPS. If you’re running it on a PC, there are 60 FPS mods with improved draw distances and shadow resolution.