The formula for determining AF = ((bets + raises) / calls ) . So an Aggression Factor of 1.0 implies that the person makes bets equal to the mount of times he calls, which is a relatively balanced and flexible marker.
In general, if you see an opponent’s AF dropping consistently below .75, you can surely consider him to be a passive player, but being too passive isn’t normally a good strategy in poker. An AF over 1.5 might be considered an aggressive player, but you still have to monitor this over a matter of time.