A renaissance fair is a place where people can pretend that they're living in an old-timey kind of world. The word "renaissance" means a time when art, science, and literature were very important, and so at a renaissance fair, you'll often see people wearing clothes like they did back then, and doing things like jousting or sword fighting. You might also see people selling things like handmade soap or wood carvings, and sometimes there are shows with music and theater too.
When you go to a renaissance fair, it's a little bit like stepping back in time, but it's also just make-believe. So you don't need to worry about anything bad happening, because everyone is just playing pretend. It's a fun place to explore and learn about different parts of history, and to appreciate some of the things that people used to do and make a long time ago.