What exactly does it mean to have o positive blood?

Blood types: In general there are 4 major blood groups, a, b, ab and o, signifying the presence/absence of certain molecules on the red cell surface. Type a has only "a" molecule; type b has only "b" molecule; type ab has both and type o has neither. The "positive" and "negative" reference refers to the presence/absence of rh molecule on the red cell surface. O positive is the most common blood group.