The "brain in a vat" thought experiment is where we imagine that we could just be brains in vats getting our neurons stimulated to make it appear as though reality was the way it was (basically The Matrix). Hilary Putnam is one person to try to find arguments against this, one of which is the rather strange language argument given in the comic, which like it says doesn't really convince many people that we aren't brains in vats per se, but makes us think carefully about how reference works in language, if we are committed to believing language references the outside world at all.

You can read more about the Brain in a Vat here and Putnam's Argument, or perhaps better just listen to him describe it, I should warn you though, he is extremely dorky.