looks like the sum of all bids or asks for each price range.
Exactly. Not very precise but it give a rough picture. What I really like about this software is that it connect directly to MtGox API so the latency is lower than the usual charting website. I use it in conjunction of bitcoinwisdom.com
Cant be, its completely random. A 5k wall has no effect, a 300 one does. Either so terrible programmed that its useless, or being misinterpreted.