it means that you feel so confident while you're on the computer, that you actually have the guts to say whatever you're feeling aloud, or rather typing, whereas if you were face to face with that person you wouldn't really say that because the consequences while you're at home and just sitting behind your computer are not nearly as severe as when you are standing face to face with somebody.
say with bathroom wall ( the old app on facebook ) people at my school ALWAYS used to diss other people they haven't even talken to but knew their personality quite well. whenever somebody did that (dissed someone) a person would respond with 'well why don't you just stop hiding behind the computer screen and say it to my face ?'
to sum it all up , its just basically what the name says: hiding (being too shy to confront someone) behind the computer screen (and writing it [sometimes anonymously] to someone [a direct message] that could be offending or embarrassing)
hope this helps :)
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