It provides an increased to your AC, will saves, and bonus HPs.
Aid is a wizard spell that gives allies fighting alongside the caster a boost of temporary health and armor. A good way to see through the simplicity is understanding what Aid does NOT provide: the Aid spell doesn’t refill depleted hit points or grant immunity to damage in any form; it doesn’t protect against hostile effects of spells cast from other magic users which are traditionally unaffected by creature Armor Class (but such effects might be suppressed if they fail their saving throw); it won’t protect an ally from disadvantage or critical hit pain or vulnerability on the Shield Defense slot—which, as a reminder, grants immunity for three rounds after taking damage; nor can it make up for the lack of Agility/Dexterity bonuses rolled into Armor Class.
