Okay, so the Dorset Ooser is a very special mask that comes from a place called Dorset, which is in England. This mask is really big and scary-looking, with big eyes, horns, and a big nose.
A long time ago, people used to wear this mask during something called a mummer's play, which was like a little play that people would perform. They would wear the mask and act out a story or a scene, to entertain people.
The reason why the mask looks so scary is because it was supposed to scare away bad things like evil spirits, ghosts, and even diseases. People used to believe that if you wore a scary-looking mask, it would trick the bad things into thinking you were one of them, and they would leave you alone.
Nowadays, the Dorset Ooser is a really special thing that people like to keep safe and look at in museums or special collections. It's like a piece of history that reminds us of how people used to do things a long time ago.