With germ cell ovarian cancer, germ cell refers to the precursor cells that develop into eggs, ovarian refers to ovary which is where the germ cells live, of which there are two that sit on either side of the uterus.
So, a germ cell ovarian cancer refers to situations where these precursor germ cells become cancerous and form tumors.
During fetal development, the entire body derives from three layers, called germ layers, the ectoderm, mesoderm, and endoderm.
These germ layers are made of germ cells, and the germ cells migrate out and differentiate into all of the different types of tissues, for example some ectodermal germ cells become cells of the brain and spinal cord, some mesodermal form bone and muscle, and some endodermal cells become cells in the gastrointestinal tract.
Some very special germ cells, however, remain as germ cells - meaning that they don’t differentiate, they remain pluripotent, meaning that unlike the cells that differentiate these germ cells retain their ability to turn into other cell types. They’re like ancient little shape-shifters.
Normally, during development these germ cells head to the ovary in women or testicle in men where they remain for decades, eventually developing into eggs or sperm, respectively.