Natural family planning is a way for mommies and daddies (or anyone who can have babies) to decide when they want to have a baby by paying close attention to mommy's body.
There are lots of different ways to do natural family planning, but one common way is to keep track of when mommy is ovulating. Ovulating means that mommy's egg is ready to be fertilized by daddy's sperm, which is what makes a baby.
Mommy can tell when she is ovulating by looking for changes in her body like the texture of her cervical mucus or by taking her temperature every morning. Mommy can then use this information to figure out when she is most likely to get pregnant and avoid having sex during those times if they don't want a baby right now.
Natural family planning takes a lot of practice and careful attention, but it can be a great way for mommies and daddies to plan their families in a way that works for them without using any hormones or devices.