The heavy, rigid paper that's used to make the boxes you use for mailing things is called cardboard. Cardboard also comes in handy for crafts and projects in classrooms.
A lot of cardboard is made from several layers of thick paper, so that it's stiff and strong, and protects items inside cardboard boxes. You can also use the word cardboard to describe a fictional character who doesn't seem real: "The mother in the movie was such a cardboard character." In the 18th century, cardboard was known as card paper.