Okay kiddo, imagine you're going to a summer camp and the camp leader tells you that in order to stay at the camp, you need to follow some rules like not hitting other kids, not running away, and tucking in your bed every morning. Now, let's say some kids at the camp start saying that the leader didn't really mean that you HAVE to follow those rules to stay at the camp - that you can still stay at the camp even if you don't follow the rules.
This is what happened a long time ago in the church when some people started saying that you could still go to heaven even if you didn't really follow Jesus with all your heart and do what he says, because he loves us no matter what. This idea was called "easy-believism."
But then, other people said that if you really believe in Jesus, you will want to follow him and do what he says. They said that it's not just about saying you believe in him, but about really putting him first in your life and trying to follow his teachings. This idea was called "lordship salvation."
So, the controversy was between these two ideas - easy-believism and lordship salvation. Some people believed that you could still be saved even if you didn't try to follow Jesus, while others believed that true believers would always try to follow him. The argument was about what it really means to believe in Jesus and what happens when you do.