At the Last Supper, Jesus gave his disciples bread and wine. Then shortly after he was crucified, he appeared to Peter and again gave him bread and wine. Jesus told Peter that whenever he ate bread during the Mass, it would turn into the flesh of Jesus inside him, and whenever he drank wine during the Mass, it would turn into the blood of Jesus. This is called transubstantiation. In this way every Christian could be part of God. And Jesus would be constantly reborn into the world.
Later on, Christian bishops developed the idea of excommunication. If people were bad, or made the bishops angry somehow, they would not be allowed to take communion (to eat the bread and wine) anymore. Without the bread and wine – without taking communion – people could not get into Heaven.
This is a video of people taking Communion