IgM stands for immunoglobulin M, one of the classes of immunoglobulins (also called antibodies) that are in the blood. IgM is an antibody that remains in the bloodstream where it can kill bacteria that enter the blood.
Low levels of IgM occur in multiple myeloma and some types of leukemia.
The 'low', 'normal' and 'high' ranges vary with age, and are also lab-specific. However, a reasonable normal range is 45-250mg/dl.