a.k.a. detonation, put simply, knocking is when the airfuel mixture in the cylinder explodes before it's supposed to and if it's bad or left long enough it can put a hole in your pistons. It depends on many factors and so there are many ways to combat it, grade of fuel, charge temperature and most importantly, your timing. There's loads of sites that explain it with diagrams and animations and everything - just do a search in google