While this is a very good explanation, I would like to add that you will see negligible functions also in other proofs. One example are peusdorandom strings. If an attacker looks at a string, he should only be able to decide if this string is pseudo-random or "real" random" with probability (distribution) $$½ + negl(n)$$$$½ + \mathit{negl}(n)$$
He can always toss a coin (that gives him probability ½) but maybe he can extract some piece of information that "improves" his guess.