1a. a covering for the face; b. an impression of a human face; 2. a respirator to be worn as a protection against poison gas; 3. a false face, as worn by children at halloween; a disguise
The first part of the definition is the one that will be mainly focused upon, but allow me to put it into my own words...
We all use masks every day to hide our true self, feelings, ideas, opinions, and many other things from people. They are often used when one is around different groups. For instance, when somebody is around family, they always act a certain way; whereas when they are around friends, they act completely different. The use of masks becomes evident when one is surrounded by two groups that they act differently around. Then, the person's personalities clash and they are acting strange by trying their hardest to find the best way to act for both groups.