It's a disease in which patches of endometrial tissue (the stuff that lines the uterus) grows outside the uterus. It can start to grow on the abdominal cavity or gets stuck to the ovaries, intestines,bladder etc..The thing is that it responds to hormones in the same way that it would in the uterus, causing cramps, pain,and eventually makes scar tissue.
These bits of sacr tissue are called adhesions & can sometimes grow to get in the way or organs or stuck tham together...that;s when they need to be surgically removed.
It can interfere with fertility if the adhesions block the passage of the egg down the tubes,and it can either cause no symptoms at all or give the woman a real dragging pain in the tum, bloating, pain associated with sex & weird period patterns.