A death mask is like a big plaster mold that is made when someone dies. Imagine playing with Play-Doh and making a shape with it. When someone dies, a special kind of Play-Doh is put on their face (which is like a big lump of clay) to make a big mold.
This mold is made by carefully putting the Play-Doh on the person's face so that it covers everything. Sometimes, they might even put straws in the person's nose so that they can breathe while they're making the mold. The mold is left on for a while until it dries and hardens.
When the mold is hard, it's carefully taken off the person's face and is left to dry completely. Once it's dry, people can use it to make a copy of the person's face so that they can remember what they looked like.
This is kind of like making a picture of someone, but instead of drawing it, you are making a 3D copy of their face. It's really special because death masks are usually made of important people who have died, like kings, queens, and presidents. It's like a way to remember someone who was really important.