I'm a doctor IRL
it makes endometrium (the layer that bleeds on periods) grow invasively out of uterus, usually on abdominal cavity but can even get to the brain... is not malignant since cells are normal but the symptoms depend on the invaded organ
the treatment might need surgery, usually laparoscopic, can come back, but can be controlled with hormones/contraconceptives, it also can cause bleeding of the invasive tissue on periods, since is exactly the same tissue that causes the periods... the infertility can be caused by the invasion of the ovaries, if it damages them