2-D Calvin is a period of time when Calvin imagines himself as a 2-D character. He claims that he is "thinner than a sheet of paper...having width but no thickness..."
He stands up straight and turns sideways so he is "nearly invisible, a vertical line!"
In the last panel, Calvin claims to his dad that he was two-dimensional all morning, which was why he didn't see him all morning, causing his dad to say "I'll bet you can't do it all afternoon."