A die-in is when a group of people lay down on the ground and pretend to be dead. This is usually done as a form of protest to show how serious they are about a particular issue. It is like playing a game of pretend with your friends, but instead of playing a happy game, you are playing a very serious one. It's like pretending to be a superhero but instead of fighting villains, you are showing how much you care about something important to you. It is important to remember that this is just a pretend game and everyone gets up and continues their day after it is done.