NFP stands for Natural Family Planning. It teaches couples to monitor the signs and symptoms of the woman's body, such as body temperature at the start of the day, to understand and track the different phases of her fertility cycle.

It can be used to avoid pregnancy, it can be used to increase the likelihood of conception, it can be used to help understand and address the root causes of infertility, it can be used to more accurately estimate a pregnant woman's due date, and it provides a barometer of a woman's overall health and hormonal balance.

It’s supposed to be hormonal cycle tracking but a lot of Catholics treat it like a birth control method (which is the same way planned parenthood discusses NFP) because they’re not actually open to life (having kids) at the moment and want to have sex anyway