In broad terms it is to conceal or cover up something from the view of others.  This can be a literal thing such as wearing a mask on you face to cover up your face, or to hide personality traits or parts of your behaviours so other people will see you act and think of you in a different way then you actually are.  Masking is something people with autism do at times to hide their different neurotypical behaviours so they will feel more accepted with people.