Does anyone know what MUGENs are?

Mugen is a freeware 2D fighting game engine designed by Elecbyte if you want to make a game, you must first know C language programming. After that you must draw characters to set a fair gameplay otherwise you have a crazy game control that no one understands. Music is also something players like to have because boring no sound game is not fun. SFX is another thing, hearing the sounds of the characters hitting enemys or doing that sweet combo is worth it.

you can get help from the community here: https://mugenarchive.com/forums/forum.php

if you want some kind of idea how Mugen works with fighting characters, go to https://www.saltybet.com/ this place has many many characters that fight. Computers control the characters to win the round.

i recommand this channel to understand gameplay of fighting, why combos exist, iframes, etc: https://www.youtube.com/c/CoreAGaming/videos

Awesome, thank you so much! 

Yes!!! I remember a time were i couldn't find the jjba Arcade game to install in my pc, then i would just install MUGEN and the JJBA charts, like Jotaro, Kakyoin and DIO. Etc. 

Nice!