TAG is a term used to describe multiplayer team modes named Tag Coop or Tag Team Vs where players try to complete a beatmap cooperatively. Beatmaps that are specifically designed for those modes are usually referred to as TAG beatmaps/difficulties.
In these modes, each player is responsible for one comboset, and throughout the song, players switch between combosets. For this reason, TAG beatmaps are designed in such a way that their combosets are placed much farther apart and more complex than usually, making it nearly impossible for the beatmap to be played by a single player.
TAG difficulties are usually denoted by the use of the term "TAG" in the beatmap difficulty name. Most often "TAG" will be followed by a number, which typically denotes how many players the difficulty is meant for (e.g. "TAG2" for two players or "TAG4" for four players).
Due to their nature, TAG difficulties are either unranked or Loved, because they are intentionally designed to be near-impossible for a single player.