The term describes how tissues similar to the lining of the womb grow in other places outside of the uterus, triggering abdominal or pelvis pain, period pain, pain during sex, pain when going to the toilet, nausea, constipation, diarrhoea and fertility issues.
Article source: https://www.huffingtonpost.co.uk/entry/endometriosis_uk_6409d256e4b01ea51224fa81