What exactly does it mean if you have a pap test that comes back showing abnormal cells?

Depends "abnormal" There is a wide range of abnormal pap smear results. Some tests come back showing you have a virus called hpv which is extremely common in sexually active women. Pap smears can also show that the cells of the cervix themselves are abnormal, showing pre-cancerous or cancerous cells. Since "abnormal" can mean so many things, you should ask your doctor specifically what it showed.
Call your doctor. The abnormal cells on that exam means you need to call and see your gyn doctor. The chapter is to big to be discussed here. Just go and see your doctor and he will explain to you all you need to know.