It's a topographical surname, meaning it originated from a place name. In this case, it likely refers to someone who lived near a masked or masked-up area, perhaps a place with a lot of trees or a wooded area.
While it's most common in England, there are also records of people with the surname Mask in other countries like Germany, Ireland, and the United States. It's important to remember that surnames can be found in various locations due to migration and family history.
To find out more about the specific origins of your own family's Mask surname, you might try researching family history websites or contacting a genealogist.