Agility reflects your Character's coordination, reflexes and speed.
Your Character's AGI Mod +1 determines the number of Actions your Character may make in a single Round of Combat
Add your Character's AGI Mod to their Initiative rolls.
Add your character's AGI mod to your Speed
Add your Character's AGI Mod to their Dodge Actions
When your Character needs to react to something, usually outside of combat, that requires quick reflexes and reaction time, they will make an Agility Check by rolling 1d20 plus their AGI Mod.
When at 0 a Character is unable to move or take any actions and immediately falls prone, to weak to move.