The Last Supper is important to Christians because it describes how people should serve others, not serve themselves or expect others to serve them. During the Last Supper, Jesus gave his disciples bread as a symbol of his body and wine as a symbol of his blood. Both of these signify Jesus' sacrifice on the cross.
The Last Supper is meant to remind people that Jesus made the ultimate sacrifice, his life, so others could live. In return, he expected nothing more than faith, and he asked that everyone remember his sacrifice. The day after the Last Supper, Jesus was arrested and crucified on a Roman cross.