Vortex Sound Research

Hi, guys.

I am just getting into synths (don't own any hardware synths yet - waiting for my next few payments to come in, maybe I'll get a Microbrute mk. I) and I am trying to learn how they work and, well, what's in the dang things! So, one thing I just CANNOT figure out about synths is what this 'filter' is. I am reading a lot about it in the literature and on forums. I tried reading a book by a guy named Chowning - that wasn't helpful. So... what are they? Sorry if this is a dumb question. I'm kind of a n00b.