A loophole is an accidental technicality or unclear section of a written document that allows someone to avoid following a rule or fulfilling an obligation. If you've discovered a way to get out of paying taxes on money you made last year, you've found a loophole.
There can also be loopholes in politics, where politicians get away with something through a technicality. And if a criminal goes free despite being found guilty of a crime, it might be because of some unforeseen loophole in the law. The original loopholes were narrow slits in a fortress or wall through which defenders could safely shoot arrows — it was seen as an unfair advantage by some, which resulted in the modern meaning of loophole.