Definition of "angle shooting
There is what is legal to do at the poker table, there is what is illegal to do at the poker table, and in between there is "angle shooting", which is a gesture that passes muster, but is not morally acceptable. Angle shooting is when it is done with malice for the purpose of gaining an advantage. When it is done out of ignorance, it is simply a beginner making a mistake, but when it is done knowingly, it is "angle shooting".
Concrete example of "angle shooting
A player bets the flop and you look at your chips. You act as if you want to raise a lot of chips. You play the player who wants to raise, without doing so, and watch how your opponent reacts. At the poker table, you must play honestly and play by the rules. "Faking" a raise or folding is borderline acceptable. You won't be penalized outright, but if you do a lot of angle shooting, you may be banned from some casinos. Think of angle shooting as the line between legal and illegal.