There are four major blood groups : A,B,AB & O.
These are antigens on the surface of RBCs.
Type O is the universal donor, meaning that this type lacks the antigens A,B,AB so it can be safely given to anyone. However, type O people can only receive O blood because type O people lack the antigens to A,B,AB giving them any of those types will cause the person's body to immediately produce antibodies against those antigens causing mass destruction of red blood cells with fatal results for the patient.
That's why we can't mix certain blood types during a transfusion or transplant.