They are called universal recipients because they can receive a blood transfusion from a person with any blood type, not just their own blood type. To put it another way, the immune system of a person with AB blood will accept blood from all possible donors, whether they have O, A, B, or AB blood.
O blood types are unique in that they have no antigens. A blood types have an antigen that is specific and unique to A blood, as do B blood types. The AB blood type means that both of the antigens for A and B blood are both present.
Since both A and B antigens are present in a person with AB blood, the recipient won't reject the blood. The body identifies that blood as "self" rather than "foreign." O blood has no antigens and is referred to as universal donor blood, so that will not cause a reaction. So, a person with AB blood has all of the antigens that are possible, and an O negative person has no antigens that would cause a reaction.