They are teaching you when you can have sex and not conceive a child without using a chemical or mechanical contraceptive.
It's more complicated than that and I'm certain I'm explaining things badly, but here's another try: A couple practicing NFP for objectively sound reasons -- NFP can be used wrongly -- is attempt to delay having another child. So is the couple using an artificial contraceptive. (We're not discussing the additional wrongs associated with abortifacient contraceptives.) But there are important distinctions. The NFP couple is open to the possibility of life and is doing nothing artificial to render it unlikely. The contraceptive couple is saying a firm "no" to the procreative aspect of the act and is introducing artificial barriers -- physical and otherwise -- to make this so.
A couple that is practicing NFP may be open to the possibility of conception but their reason for practicing it is to prevent conception. This goes against the Catholic church's teachings that "every sperm is sacred".
And it is very possible that women can still go through their un-medicated menstrual cycle even if an egg is fertilized.