In simplest terms, an anti-magic aura is a field that absorbs magical energy, usually by redirecting it into making that field stronger. This can take the form of a personal shield or a magical barrier.
Anti-magic auras are not considered "spells" in the conventional-sense, and instead come from the known as auromancy. Only a skilled Auromancer can produce effective anti-magic auras. It is considered the height of the skill.