Yes, as a respected Mugenite, I am bombarded with that question all the time.
Mugen (spelling variations: MUGEN, M.U.G.E.N.) is a freeware - I repeat - freeware* - 2D Fighting engine that allows you to create dream matches by creating and downloading characters and stages. It's totally customizeable, from screenpacks (Main Menu, Character Selection Screen, Versus Screen) to lifebars (In-game HUD), all available for download.
Because Mugen has been turned into a community, however, there are rules that Mugenites must follow to make the community safe and enjoyable. Here are some of them.
1. While creating characters, stages, or lifebars of your own, don't try to take people's coding and sprites unless you have permission. This doesn't apply to open-source creations. Also, when a creator tells you you can use their material, but you have to credit them, please do so. In fact, always credit those that helped you make your Mugen creations in your readme.
2. You cannot send your modded versions of the Mugen creations of others, let alone release them to the public.
3. If you decide to join a Mugen Community Site such as Mugen Infantry or the Mugen Fighters Guild, please abide by their rules. If you don't, some features of the sites mught be taken away, and your account might be suspended, or even banned. In other words, don't be a fcuking dumbass.
Since I don't want to be bombarded with basic Mugen questions, I recommend going to txrocker281's YouTube profile for tutorials.
*I say "freeware" because some people think you have to pay for it.