What is leukopenia? Doctor said it's only slightly low and not to worry about it, since I don't have any symptoms.

Low WBC. Leukopenia means that there is a decrease in the white blood cell count. White blood cells are the blood cells that are part of the immune system and responsible for fighting infection. There are different types of white blood cells that can be low. This is something that needs to be monitored. If it persists or is associated with other changes in your blood counts it needs further evaluation.