What exactly does it means when your rubella test comes negative?

No immunity. If you talking about rubella antibody test, it means you did not have rubella, you did not have vaccine for rubella, or if you had any of above mentioned exposures you do not have any immunity/resistence against this disease. If you had disease or vaccine for it it would be expected that you have antibodies in your body for this disease if your body responded properly to it but not 100% of the tim.