Agility, or AG is one of the SPECIAL skills for your character.
Agility and AP[]
Agility mostly effects your Action Points, or AP. More Agility means more AP, which means being able to do more actions in one turn.
With an AG 3, you have 6 AP
With an AG 4, you have 6 AP (don't set it to 4)
With an AG 5, you have 7 AP
With an AG 6, you have 8 AP
With an AG 7, you have 8 AP (don't set it to 7)
With an AG 8, you have 9 AP
With an AG 9, you have 9 AP (don't set it to 9)
With an AG 10, you have 10 AP. This is the maximum amount of AP.