This is a lazy response but rather than re-type all this, easier to just point you HERE. (TL;DR create custom MP maps/edit maps/build new ways to play Halo MP)
This thread was added on October 10, 2019, with posts from ske7ch343.
Original Post
https://www.reddit.com/r/halo/comments/dg50te/can_someone_explain_to_me_what_is_forge_what_does/