what is mod
from https://osu.ppy.sh/community/forums/topics/726725pishifat wrote:
A "mod" is advice/feedback intended to improve a beatmap. Each beatmap is linked to a discussion page where anyone can detail their concerns and a beatmap's creator can respond. For example, this is a discussion page with a few dozen mods. Mods are a necessity for improving and pushing a beatmap towards rank!