If you click on the WTF on the upper right corner you get explanations for all of the graphs. But not yet for the bars being below or above zero. The author will add that soon (2013-03-23).
Meanwhile you can look at a reddit question here: http://www.reddit.com/r/Bitcoin/comments/1aqtgo/a_note_for_newbies_about_market_forces_feel_free/c8zz2ro
- green bars above the axis represent increases in buy/bid orders
- red bars below the axis represent decreases in buy/bid orders
- red bars above the axis represent increases in sell/ask orders
- green bars below the axis represent decreases in sell/ask orders
(I think you can replace axis and above/below by current price and right/left, then it is correct. Though this does not explain the bars above and below the line parallel to the x axis)
Well, I didn’t completely understand, why they are above and below, but I understood from the WTF explanation on the site, what red and green means. tl;dr: Green has different meaning, if it is left of the current price or if it is right of the current price. Same for red bars.
Maybe comboy, the author of that site, adds a good explanation for this soon (he promised to do so on freenode irc chat).