Ovulation is the moment an egg cell ruptures and is released from one of the ovaries.
Within 48 hours the egg cell needs to meet a sperm cell for conception to take place. If this does not happen the egg cell dies off.
It takes a month before the girl is fertile again.
Ovulation takes place in between two periods, at midpoint of a girl’s menstrual cycle.
With a girl who has a cycle of 28 days, ovulation takes place on day 14 (starting to count at the first day of her period).
If you have sex without using birth control, you are most likely to become pregnant right before or just after ovulation. There are couples that don’t have sex halfway the menstrual cycle, trying to avoid pregnancy. But this ‘calendar method’ is not very reliable.