Kudosu is a reward system for modding. It awards points for successful mod posts in beatmap discussions. Accumulated points can be seen on a user's profile page.
Kudosu is awarded to mod posts that are helpful and point out useful suggestions for the beatmap. If a mapper believes that a mod post is useful and addresses a valid issue, they can click the thumbs up button and award kudosu to the user who posted the mod.
In forum modding, kudosu used to be exchangeable for star priority.
Nowadays, kudosu is used for the evaluation of a user's capability to become a Beatmap Nominator. A user must have 200 kudosu in osu! or 150 kudosu in osu!taiko, osu!catch, or osu!mania in order to apply.
The word kudosu, sometimes stylised as kudosu!, is a portmanteau of kudos and osu!.