It acts in three ways:
1.      Thickening the mucus at the cervix, blocking semen (spermcells) from entering the uterus.
2.      Preventing ovulation. When there is no monthly release of an egg from the ovaries, there can be no fertilization.
3.      Changing the lining of the uterus, so that even if an egg would be fertilized implantation would be unlikely.

Because oral contraceptives act in three different ways it is a very reliable method of birth control.