Aid strengthens and sustains all creatures. It heals fatigue, exhaustion, suffering, wounds, negative levels—any non-incapacitating condition from which one can still receive a benefit from previous kills.
Aid also grants temporary hit points to living beings and damages undead beings on contact The temporary hit points persist as long as they continue to take damage while in effect. All damage being absorbed by the recipient is translated into temporary hitpoints that don’t affect any other creature or objects until destroyed at end of combat or until stopped via Dispel Magic – these amounts are applied on top of existing HP/hitpoints for existing creatures (with exception to constructs), ANY incoming attacks will have an effect equal to the damage negated by these temporary hitpoints.
For instance: Aid is cast and a creature has 15 hp and is currently at 0/10 (fatigued) – now it will have 45 Temporary hp before those are completely gone until they rest or recieve healing outside of aid. Any attack done to them now during this time period will be negated by the temporary hitpoints and they would receive half damage.