Covenantal nomism is a big fancy term that describes how people thought about getting into Heaven a long, long time ago.
You know how when you have a best friend, you make promises to be kind to each other and keep each other's secrets? Well, imagine that God is like a best friend, and people who follow God are making promises to be kind to others and to love God. This promise is called a covenant.
Now, everyone knows that sometimes people mess up and break their promises, so the people of this time believed that there were rules to follow to make up for breaking their promises. They believed that by following these rules, they can make things right with God and be able to go to Heaven.
See, covenantal nomism says that God made a covenant with the people He loved, and even though people might not always keep their promises, God still loves them and wants them to keep trying. So, when they break that promise, God gives them rules to follow to make things right again.
It's like if you tell your best friend that you're going to clean your room, but you don't do it. Your friend might be upset, but they still love you and want to be your friend. They might give you a list of things to do to make it up to them, like buying them ice cream or helping them with their homework. And, if you do those things, your friend will forgive you and still be your friend.
In the same way, covenantal nomism says that God loves us, even when we mess up. And, He gives us rules to follow to make it right again. If we follow those rules and try our best, God will forgive us and let us go to Heaven.