Osu!mania is a rhythm game in the osu client but revolves around hiting objects on a lane rather than involving your Mouse. It is by far the most stamina demanding out of all of the gamemodes.
unlike other gamemodes, mania has its own seperate system as you can use more than four keys which is the standard amound. The most popular out of the extra keys being 7 keys. It is recommended to play 7k as it has its own leaderboard for it.