A poacher is someone who breaks the law to hunt or fish. If you see someone hunting deer in a state park that's got "No Hunting" signs posted on trees, that person is a poacher.
If you don't follow the rules and laws about hunting, but grab your shotgun and head into your neighbor's yard to shoot rabbits, you're in danger of being a poacher. Someone who captures wild animals illegally is also a poacher. A completely different, much nicer kind of poacher is the one you use for poaching eggs — this type of poaching consists of gently cooking food by submerging it in simmering water.