Please explain what it means to be rh negative?

Lack D-antigen: "D" is a protein in a particular configuration that's present on many people's red cells, but missing on some. It's genetic, from birth. There's nothing really "negative" in any bad sense about not having it. However, folks who lack it make antibodies that destroy red cells (transfused, unborn child's) that do have it. It's a consideration in blood banking and pregnancy.