What does it mean when someone tells you that you have low blood? Not blood pressure but low blood.

Anemia, possibly. When not referring to low blood pressure, "low blood" can refer to anemia, deficiency in the amount of hemoglobin in the bloodstream. Hemoglobin gives blood its color and carries oxygen.Symptoms of anemia can include fatigue. Causes of anemia include blood loss (periods, GI bleeding), bone marrow problems, inherited abnormalities, sequestration in the spleen, red blood cells bursting (hemolysis).
Low blood. When someone throws out a term like 'low blood' you should directly ask them what they mean. Very often they don't know what they are talking about. As 'low blood' isn't a medical terminology, we can only speculate what 'someone' meant, and that isn't likely to help.