Angling is any time that the player acts out of turn. They could be folding, betting, or checking early. If it is not that player’s turn to act, then it is against the rules to do so. Although angling is not directly against the rules in most casinos because it happens all of the time by new players. But when somebody is consistently breaking the rules or unethically angling, the floor manager and the other players may get involved.
There are two basic ways to angle.
Folding out of turn by making a motion as if they are going to fold their hand, or actually folding their hand, and then reclaiming it when the action truly gets to them.
Betting out of turn by making a motion as if they are going to make a bet. Players to the right of that player see this and change their behavior because of the signal. When the action reaches the angler he pulls his bet back and does not truly bet at all.
Most players only use angling in a spiratic fashion and are not severely punished for doing so. However, there are players who angle on a consistent basis. These players are formally known as angle shooters. If the house rules permit the actions of an angler then angle shooters may have their way.
If you have an agler at your table and you disagree with their actions, then tell your dealer or have a floor manager come over and talk to him. It might do you some good and it might not, but at least its worth a try.