The word chemotherapy is composed of two segments: Chemo- and therapy. Chemo-something is something which has to do with chemical(s). Therapy is a word on its own. It is pronounced: kí*mó*thè*ruh*pí
It is a method of treating cancer. The idea is that you give the patient enough poison to kill the tumor but not the person. This tends to have very serious side effects. The most outwardly visible, but probably the least important, is hair loss.