For example, right now the green line is much taller than the red line. The red line is almost flat extending out to the right. If you don't know what I'm talking about, I'll get a picture and circle it.
Green line is the bids on the order book, red line is the asks on the order book. Basically the height of those curves at any particular price represents the cumulative sum of the bids or asks above or below (respectively) that price.
So the "flatness" means nobody is trying to sell coins above a certain price (red line), but lots of people are trying to buy them for cheap (green line).
Excellent. Thank you very much.