Aggression Factor: is the ratio of how often an opponent will bet/raise, divided by how often he calls.
So it can be a number between 0 and infinite.
And it usually will be a number between 0.5 and 5 (0.5 being a very passive player... 5 being a very 'aggressive' player.)
How to use this number. Or how to exploit an opponent whose AF is too high/low:
When it's too low (<1) the opponent is not aggressive enough, which means he will usually call with his drawing hands... or check behind. He only bets when he has a made hand. He does NOT: raise for information, raise with a draw, donkbet, probe-bet, river-bluff, bluff in position (when it's checked to him), float (to steal the pot from you on the turn), he doesn't cbet enough, he doesn't triple barrel bluff, etc.
So valuebet your made hands against him. When he keeps calling, he often will have a draw (or marginal hand). If he makes a crazy bet (or raise), respect his bet.
When it's too high (>3) the opponent is capable of all those moves.
So often let the opponent do the betting for you when you have a made hand. (for example keep check-calling, let him bluff in position)
If he makes a weird raise, he can also do that with a draw (or marginal hand)! So adjust your game accordingly.
Also create a popup: with Aggression by street so you can see his
Flop Agg Factor: Aggression Factor based on the flop only
Turn Agg Factor: Aggression Factor based on the turn only
River Agg Factor: Aggression Factor based on the river only
because a lot of the OVERALL aggression stat, will consist of the 100% Flop continuation bets, when the opponent is a lot LESS aggressive on the turn/river. While other opponents are notorious for their river bluffs.