Agility grants the player a 7% increase in base movement speed. For example, without the perk equipped and holding an SMG or a Shotgun, you will move at 100% of the base speed, so when Agility is equipped this will rise to 107%.
Agility is a good perk to use with any weapon class, giving players using SMGs, Shotguns or the Combat Knife unrivaled speed, while making movement with LMGs, Sniper Rifles or the Riot Shield more tolerable. Though the 7% speed increase doesn't sound like a lot, it is a noticeable difference in game.
It is a very good idea to pair Agility with Marathon, as Agility perfectly complements the unlimited sprint provided by Marathon.