When a woman is fertile it refers to the time in the woman's menstrual cycle which precedes the ovulation days.
When a woman is fertile you can have sex with a man but your chances of getting pregnant are low compared to the days when you're ovulating.
When a woman is ovulating the ovulation is a process where the woman's egg cells are mature and being produced from the woman's ovary.
Ovulating days during a woman's menstrual cycle are the best days for having a high chance of getting pregnant with a baby.