Well, imagine there is a really important person named Jesus who is loved and followed by many people. Jesus is considered to be the savior of the world and is special to Christians.
One day, some people didn't like Jesus and they arrested him. They then brought him to a trial where they accused him of doing bad things. However, even though Jesus didn't really do anything wrong, the people who didn't like him said he did and wanted him to be punished.
So they put a crown of thorns on his head, whipped him, and made him carry a cross. They took him to a place called Golgotha and nailed him to the cross. It was very painful and Jesus suffered for hours on the cross until he passed away.
This event is called the crucifixion, and Christians believe that Jesus died for their sins. They celebrate this event every year on Good Friday. The "Christ i" you mentioned is actually the shortened version of "Christology," which is the study of Jesus Christ and his role in Christianity.