Agility, or AG is one of the SPECIAL skills for your character.
Agility and AP Edit
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.