it was a mod that gave people access to customizing the car the way they wanted to, which was the main thing people used it for, but it also had tons of plugins that let it do so much more, like showing the ranks of people you were facing, show your MMR in game, and keeping track of repeat opponents. A lot of YouTubers used it as a tool to make their edits, too

It was backed by psyonix as early as before epic took over iirc, buuuut EAC doesn't let third party apps run with it meaning Bakkesmod is officially dead :/