What is cervical mucus and role concerning fertility?

“Cervical mucus is the mucus that is the plug inside your cervix. Usually, it’s thick and it’s preventing bacteria from your vagina, from getting into your uterus so that you don’t develop all these infections, but in the middle of the month when you’re ovulating, it needs to become thin and slippery and stringy so a little sperm can get in and allow you to have a baby.”