for example "Everybody [is] joking about him" what does [is] stands for ?
first of all I'll let you know that this is not part of any English grammar.First question is that "where have you found this one? ". Look in there there might the case that someone might have written
Everybody joking about him.
That is incorrect grammatically.I mean he might have forgotten the is. To suggest him the corrector has made  covering is referring he might have written
Everybody is joking about him.
And the second one is correct grammatically as well.
I'm not 100% sure but this looks like an addition by the author.
For example, suppose some politician is giving a speech and a reporter is writing about it, the reporter might add square brackets to add information that would be obvious to anyone listening to the speech directly (in context) but is necessary when just reading the quote like "He[the prime minister] should do his job." the  are used to clarify the statement.
In your case it seems like an addition to provide proper grammar, which occurs often when quoting colloquial speech where a sentence can be understood, like "Everybody joking about him", but to make perfectly clear what is being said the "is" is added by the author.