I'm 66 And Have End Stage Renal Failure. Kidneys Have

I'm 66 and have end stage renal failure. Kidneys have stayed around 20% for about 4 years now. After a visit with my renal doctor, she prescribed ferrous sulfate (one 325 mg tablet 3 times a day). She never mentioned this to me at all. I just now found out about it when the UPS man delivered some meds to me. What is this for? And why did she prescribe it without telling me about it?