It's a really big music/arts festival and very popular. Quite leftfield. I'm not sure what music they play however it's been happening for decades and has its own traditions.
I think it can be quite expensive. Plus a lot of work since you have to prepare stuff well in advance. As far as I know, you can't just pitch up with a sleeping bag like at Glastonbury.
There are music festivals in PL of course, most of them very mainstream however there's one, somewhere north of Warsaw, which is the nearest thing to that. I forget its name.
satanist festival where all the filth and sodomy
Hard to know where you pulled that gem from.