Reticulocytes are immature red blood cells (RBCs). They're made in the bone marrow and are released into the bloodstream, where they circulate for about 1-2 days before developing into mature red blood cells. Normally, only about 1% of all red blood cells in the bloodstream are reticulocytes.
A reticulocyte count measures how many of these immature red blood cells are in the bloodstream. This gives an idea of the rate at which these cells are made in the bone marrow