So you've been conversing online. You make a quick joke. And suddenly, everybody acts like you are the greatest, meanest Jerkass ever. You get exiled, your spouse wants a divorce and all your friends leave you. And you know why? [[TwistingTheWords Because people had taken your joke seriously and felt genuinely offended. And you can't blame them for that. It's pretty easy to mistake jokes for seriousness online.]]
And to prevent these kinds of misunderstandings, there is Humor Mode. With this, you can mark a text to show that you are, well, just joking. You were not serious at all about what you just wrote.
Contrast with SincerityMode, when you don't mean a joke, and SarcasmMode.
Compare and contrast with JustJokingJustification (as in using this doesn't always work).