The lining of your uterus is made up endometrial tissues. During ovulation the endometrial tissues builds up and thicken to prepare for pregnancy. However if no pregnancy occur, they will breakdown during menses and exit the body.
Endometriosis is a condition where this endometrial tissues grows abnormally outside the uterus. These endometrial tissues can get attached to ovaries, pelvic ligaments, fallopian tubes or even other abdominal cavities such as the bladder, bowels etc.
When these endometrial tissues that are outside the uterus bleed during menses, the blood becomes trapped and unable to escape. This leads to inflammation and severe period pain. Over time, they accumulate causing the development of scar tissues also known as adhesions.
Adhesions are fibrous-like bands causing structures and organs to stick to one another. When these structures and organs abnormally stick to one another, it causes obstruction of regular function of an organ, such as block the fallopian tube or even displace the entire organ.