Mods, in Celeste, are modular game add-ons made by players that replace vanilla or implement new content. They are installed through the Everest mod loader, which can itself be installed via Olympus. Mods are hosted on GameBanana. There are over 2000 mods in existence.
Celeste mods are divided into many sections. The most popular section is Maps; those mods predominantly introduce new playable content to the game. They can be a stand-alone map (Everlasting Farewell), a remix of Celeste's campaign (Glyph), or a collab (2020 Spring Collab). Other sections include helpers for mods that add new mechanics, and skins (skin mods) for mods that replace visuals of Madeline or other characters and objects.
Modding as a whole is heavily discussed on Celestecord. A dozen channels are dedicated to modding there. Moreover, several channels and threads are available for individual mods.