There are different reasons why people use masks. A person can use a mask so others do not know who they are (as when committing a crime). Or, a mask may keep someones face safe from harm (like a gas mask so harmful gases do not get into the lungs).
There are many practical uses for masks in everyday life. Mostly, the masks used are semi-masks which cover only the bottom half of the face, the mouth and nose. An example is the use of masks in operating theatres, and by wood-workers. In surgery, the purpose is to keep bacteria off the patient; in wood-working the purpose is to keep wood fibres out of the lungs.
In Venetian masks only the top half of the face is covered, to prevent recognition. Ice hockey masks, used in the Hannibal Lecter movies, cover the whole face to protect it from the hockey puck moving at high speed.
The word 'mask' might have come from other languages. The French masque, the Italian maschera, the Spanish máscara, the Latin (not classical) mascus/masca (meaning "ghost") or the Arabic maskharah (meaning "jester" or "man in masquerade") all use the same word.
One of the oldest masks found is the Death Mask. This stone mask is 9000 years old and is probably the oldest mask in the world. It is kept in Musée de la bible et Terre Sainte, Paris.
There are also three masks from the same period, kept in The Israel Museum, Jerusalem.
Masks are used in many magical or shamanistic rituals in non-western societies. The celebration or appeasement of animistic spirits takes place with dance and song in ancient performances in many parts of the world. In these events performers use costumes and masks to mimic the supposed spirits.
There are some other uses of the word 'mask', which arise by extension. That means using 'mask' as a metaphor. In film a mask is a screen which cuts out part of the image. In social theory, a mask is the behaviour a person takes on in his role or job. Thus policemen, head teachers, army commanders, prime ministers act as they are expected to act (most of the time). The inference is that behind the mask is the 'real' person. Their role is a mask. As a verb, 'to mask' means to cover or hide.