The theology of the cross is when people believe that Jesus of Nazareth is the son of God and that he died on the cross for everyone's sins.
When this happened, it was very hard and painful for Jesus, but he did it because he loved everyone so much that he didn't want them to be punished for their sins.
This means that even though Jesus was innocent, he was punished as if he was guilty so that everyone else could go free from their guilt.
So, the theology of the cross means believing that through Jesus' death on the cross, we can have forgiveness for our sins and have the hope of eternal life with God.