Imagine you have a toy that you really like playing with. You want to share it with your friend, but you both want to play with it at the same time. What do you do? You can make an agreement or a joint declaration where you decide to take turns and share the toy equally.
Similarly, the Joint Declaration on the Doctrine of Justification is an agreement between two big groups of Christians, the Catholics and the Lutherans. They have different beliefs about how a person can be saved and go to heaven, just like how you and your friends might have different ideas about how to share the toy.
The agreement says that both groups, Catholics and Lutherans, believe that people are saved by faith in Jesus, and not by their own good deeds or actions. This means that they both agree that it is not enough to be a good person or do kind things, but that you also need to believe in Jesus and trust him to save you.
The Joint Declaration on the Doctrine of Justification is important because it helps the Catholics and Lutherans work together, and it shows that even though they have differences, they can still agree on important things. Just like how you and your friends can still have fun playing with the toy, as long as you agree on how to share it.