The bone marrow is the place where all blood cells are manufactured and released to the blood stream.
Myelofibrosis means that the bone marrow becomes fibrotic and is no longer a suitable place for blood cells to be produced.
So, our body used other organs, such as spleen or liver to produce blood cells.
However, in these organs other than the bone marrow blood cell productions are not efficient enough to supply the blood cells to meet the body's demand.