'Ovulation' is the process of an egg being released from an ovary within the woman's reproductive system and this happens 12-16 days before the next period starts. A woman is at her most fertile on the day of ovulation and the day before.
The 'fertile days' are all the days during a woman's menstrual cycle when she has the ability to become pregnant if she has unprotected sex. Due to the fact that sperm can survive for several days in a woman's body during her fertile time, a woman will have typically 6 fertile days each cycle, the day of ovulation and the 5 days before that.