Have you ever heard of money? It's what people use to buy things like toys, clothes, and food. Nominal income means how much money someone makes in a year without adjusting for things like inflation (when the prices of things go up).
A nominal income target is when the government or the central bank (the people who control money) sets a goal for how much money people should make in a year. They want to make sure that people have enough money to buy the things they need and want, without things like inflation getting in the way.
Think of it like a game. The government or central bank is like the coach and they want to make sure their team (the people) wins. They set a target of how many points (money) they want their team to score. If their team reaches that target, everyone is happy and can buy the things they need and want. If they don't reach the target, the coach might need to think of a new strategy to help their team score more points (make more money).
Overall, a nominal income target is a plan to make sure people have enough money to live their lives and buy the things they need and want.