In low-fantasy, magic is rare: the gods are distant, the fey are in retreat and superstition is widespread. Non-human characters with magical abilities are the exception, not the rule.
A unique background represents a rare or special origin that gives your character access to magical races or classes (or feats) in a way that is sympathetic to low-fantasy campaigns.
Unique Backgrounds introduces variant rules for low-fantasy characters. It is free/pay-what-you-want so that other Dungeon Masters Guild publishers can create their own unique backgrounds.