It's a platform for playing and creating video games. The stated intent was for kids to make games for kids, but as soon as game creators could make real money from it, professional devs took over. The in game currency is called robux, and it seems like every game is littered with buttons to get a higher jump or faster car or whatever for a few dollars' worth.
Because young kids are not savvy consumers, the platform is chock full of identical games with the same name and logo hoping to steal players stay from whichever game is on trend at the moment.
There are certainly some games in there worth playing, but it's a very small minority.