Ferrous bisglycinate
Ferrous bisglycinate is an iron salt in which iron is bound to the amino acid glycine, a chelated form used as an oral iron supplement. Iron is the core of haemoglobin, so supplying it lets the marrow rebuild red blood cells and refill the body's iron stores. It is used for iron deficiency, with or without anaemia.
Iron is absorbed best on an empty stomach, and vitamin C taken alongside improves uptake further, while tea and coffee bind iron in the gut and blunt absorption. Calcium carbonate with D3 competes with iron for the same absorption pathway, and iron interferes with levothyroxine, so those are kept well apart in the day. Response is tracked through ferritin, haemoglobin and transferrin saturation rather than by how one feels, since stores refill slowly after the blood count has normalised. Dark stools are expected and are not bleeding.
Persistent stomach pain, vomiting, or an inability to tolerate iron at all is worth reviewing, as is a thyroid condition that becomes unstable after iron is added. Iron deficiency that does not correct on treatment, or that returns after it, needs a doctor to look for the cause of the loss.
Reference information, not medical advice. Discuss any change with your doctor.