An allegation is an accusation, which is sometimes true and sometimes not. If you say your sister stole a candy bar but you don't have any proof that she did it, you have made an allegation.
In the legal system, an allegation is a formal claim against someone. It sparks an investigation that leads to someone being proven innocent or found guilty. In everyday use, however, an allegation is what someone thinks, whether or not there is proof. For example, if someone makes an allegation that you're a terrible driver, they might base it on their own experience, but they won't be able to prove it.