What exactly does it mean to have a low white blood cell count?

Leukopenia: Wbcs either being destroyed by virus or severe bacterial infections; medication such as chemotherapy, some antibiotics or diuretics; overactive spleen. Or underproduction as a primary effect as in aplastic anemia or suppression of production of wbcs by bone marrow as in some cancers & leukemias, overwhelming infection, hiv/aids, some parastitic diseases, radiation therapy & malnutrition.