Aid is a spell that can be learned at Level 1 and it offers two different benefits depending on which edition of D&D you’re playing.
In 5th Edition Dungeons & Dragons, when cast on a creature, aid will heal the target for one quarter to one half (1d4 +1) their maximum HP. Aiding yourself will do the same to your HD only, regardless of how many hit dice you have left.
Aid is an instantaneous conjuration spell with no components that heals as per a level 2 cure wounds spell (succeeding on a healing check treating any racial bonuses as equipment). The subject’s Constitution scores are considered 8 points higher than normal while this spell is in effect.