At first I was assuming you were being sarcastic when asking about the balk. But if you truly want to know; there are over a dozen criteria that qualify as a balk. You can basically summarize it as "an offensive player (usually, but not limited to a baserunner) allowed to advance 90 feet (one base) due to the pitcher not having a fluid, uninterrupted motion towards the plate. Sorry for the vague definition, but the rule itself is pretty vague and it is often debated upon what truly constitutes a balk.