M.U.G.E.N is a freeware engine initially designed for creating 2D fighting games. Due to it's customizability it's more commonly used for creating individual elements of a fighting game like characters, stages, lifebars etc. to the point that there are more individual creations made for the engine than actual games created with it. More info hopefully would be provided at at the article dedicated to the engine itself.
SaltyBet is a website where you can gamble away your (fictitious) money on fighting game matches. When major fighting game tournaments like EVO are going on, usually you can bet on those. However, when they're not, you get a 24/7 stream of M.U.G.E.N's AI matches, set up so between the matches there is a betting phase.
Why call the wiki after the streaming site when it's mostly about the engine?
Because as of the article's current revision it is the source of increased public exposure to the engine.
All of that sounds strangely interesting, how can I join in?