For me, masking would mean: suppressing my emotions, opinions, impulses and instincts in order to appear friendly and sociable. For example, if someone asks me whether I like rap, my natural instinct is to go into a long monologue as why I don't like it, but instead, I just say, it's not my kind of thing.
Or someone mentions flute. I immediately think of Jethro Tull and how much I like their albums, which ones I enjoy most, or the Hungarian musician Attila Kollar and Solaris, and his Witchcraft albums. Then I need to stop myself to remember that they aren't after the details or that the conversation was about something else and I'm going off on a tangent.
I have to monitor and watch my facial expressions, make a conscious effort to make sure that what I say is 'safe' or socially appropiate, try to anticipate how a person might react or feel. For example, when I'm told that I can't do something I want to do, or that my schedule has suddenly changed, I need to try not to let my frustration show, and I try to make the right noises or say the right thing (I'm not good at this).
I need to suppress my instinct to run or flap my hands or stim, especially when dealing with clients.
NTs also need to mask to some extent, but I experience stress, anxiety and other emotions very tensely, and I don't process incoming social information - other people's facial expressions and emotions as fast as they do, so it's much harder - a lot more work and exhausting.
I'm not very good at it. As soon as I'm stressed or something changes, the mask slips. I suppose that's why I got the diagnosis in the first place - I tried, and I failed.