Okay kiddo, Nisba is a fancy word that people use to describe the way that people's names can give us information about them.
Imagine if your name was Lucy and you came from New York City, your nisba or onomastic would be "Lucy from New York." This little bit of information tells us something about where you come from, and it can also be a way for people to identify or group people who are similar in some way.
Sometimes people use nisba to show their family lineage, like if they come from a long line of a certain tribe, they might add the tribe name to their own name like "Ali ibn Abi Talib bin Hashim" to show that they come from the Hashim clan.
So, in short, Nisba tells us more about a person's identity or origin, and can help people to better understand and identify with one another.