Folic acid is a nutrient necessary for making red blood cells (RBCs). So people without enough develop Anemia. As described in the link, hemoglobin is low, and RBCs are larger than normal. The latter is measured by a number “MCV”; large means >100, oftentimes around 110.
Fortunately, folic acid deficiency is very rare in the U.S., because since around the year 2000, it’s been added to flours and grains (“fortified”). The only people still at risk, who should take folic acid supplements, are pregnant women, alcoholics, some elderly persons without access to enough food, and people taking certain uncommon medications.
Women who want to become pregnant should also take folic acid supplements, because studies have shown the vitamin prevents a common type of birth defect. They should begin three months before they try to conceive.