First Aid is a Tactic that is unlocked when you first build the War Academy. It heals troops within a certain radius from where it is deployed.
Since it heals by % of max hitpoints instead of by a fixed amount, it is effective on high-hitpoint units like Generals and Heavy Cavalry but does little for troops with low hitpoints like Ranged Infantry.
The effect of First Aid is stackable. Example: If you have 2 level 1 First Aid and use them at the same time on the same target, it will be healed for 6% per heal instead of the normal 3% per heal.