The Fourth Wall is the imaginary barrier between stage or film performers and their audience. The expression "breaking the Fourth Wall" refers to when an actor breaks this imaginary barrier and speaks to the audience.
If you're familiar with the 1980's film Ferris Bueller's Day Off, you've seen this device when Matthew Broderick directly addresses the camera (and by extension the theater audience) throughout the movie.
Why not use this technique in your next blog post? If used properly and with a sense of humor, you'll instantly grab your readers' attention. Here's an example from a well-written blog on marketing and copywriting:
What if I told you I could sell anything in one paragraph or less?
(Stop glancing skeptically at the screen. I can see up your nose.)
This device works because it violates readers' expectations. You've got their attention now: it's up to you to keep it.