Natural family planning is basically when a husband and wife have sexual intercourse without conceiving a baby - but not by using a condom / artificial method.
The Catholic Church teaches that there is an inseparable connection between sexual intercourse and conceiving children, and that it is wrong for human beings to use artificial methods to break this connection.
Therefore, a way for a catholic to get around having sexual intercourse without having a baby, is by doing "Natural family planning", as they believe this doesn't go against God's teaching, since there is no artificial aspect involved.
If you don't know what natural family planning is, its basically that there are certain periods of time that a woman is not that fertile (she can have sex without having a baby) - so instead of using artificial methods of contraception, they can just have intercourse during these times ( https://www.nhs.uk/conditions/contraception/natural-family-planning/ )
So thats what I would keep in mind when answering the question.
Some "hardcore catholics" would still say natural family planning is still not allowed because sexual intercourse should generally be done with the intent of having a baby. "It would be wrong to use family planning because a couple would rather spend their money on a new car than caring for a child."
I know its a lot of reading, but heres some good information for that question:
https://www.bbc.co.uk/religion/religions/christianity/christianethics/contraception_1.shtml
If I was you, I would skip to the subheading where it says "Catholic - natural family planning" then read on from there.