Illegal fishing is a black market crime where people fish where the are not allowed to. In many oceans around the world, there is a limit to the number of fishes that fisherman may take. Illegal fishing happens when they overfish and take more then they are allowed to.
Wikipedia calls illegal fishing by another name. It is also called "illegal, unreported and unregulated fishing".
It is bad when people do this environmental crime because if we run out of fish, then future people will not be able to have fish.
It is important that fisherman around the world follow the rules of the water and not fish more then their share. That is the only way that people will still be able to eat fish in the future and that our world stays beautiful.