An example of using an angle could be pretending to fold when you are actually about to raise to encourage a call from another player. It could be deliberately misleading the other player. For instance, verbalizing a hand lower than what you have to encourage the other player to call you. In casinos, if this happens and the angler wins, then the dealer and floor manager will decide whether the win is valid or not.
Angling is more common in live poker than online due to the protection of the software, though it could be incorporated into the chat. Although it is not illegal, it is considered unethical and unfair, and will not make you very popular with other players.