I think that it means that most women are fertile for around 3 days per month, but give 6 days as sperm can survive for 5 days in EWCM and there is still a chance that the egg can be caught 1 day after ovulation.
e.g. You had fertile CM on Monday and had sex. You didnt ovulate until the Saturday, and didnt have sex any nearer ovulation but you still got pregnant because the sperm had survived until you ovulated. Its probably rare but it could happen.