In the comic:
Someone is telling Black Hat a boring story, so he uses a formatting character to reverse their speech, making them sound funny.
The (text) character inside the comic:
Text can be encoded in different ways. ASCII is a common method, and has 256 different characters, including the ones you see when you try to open a binary file using Notepad. There's an expanded encoding called UTF, which has all kinds of characters, some of which aren't actually displayed themselves, but are used to change how other characters are displayed.
The UTF code in the comic, "U+202e" is a single character which changes the text following it from "Left To Right" (the default, that you are reading now) to "Right To Left" (nettirw si txet siht yaw eht). Part of the joke is that it works even outside of the computer, part of an ongoing theme of computers (and computer programming especially) having supernatural powers.
The joke on the website:
If you examine the source of the page, you'll notice that the page title and the title just above the comic are written as "LTR" but they come out "RTL" when viewed in a browser. This is because of this same UTF code inserted just before the text. In some browsers this may cause errors. In any case, it makes talking about the comic difficult because if the HTML viewer does not "render" the text reversal you will see "LTR", whereas if it does render it, you will see "RTL".