Low maintenance as an adjective means that the person you're referring to doesn't need too much attention from her partner. If someone calls his girlfriend 'low maintenance', he means that she is 'chilled out(relaxed)', independent, doesn't really bother if he is not with her all the time or doesn't take it seriously when he doesn't surprise her with gifts on special occasions(these are just examples).
High maintenance is the right opposite. When someone calls a woman 'high maintenance', he means that she expects a lot from her companion/partner and her relationship. Such women are generally hard to please for they need their partner's love and affection to be explicitly displayed all the time.