If I go into the stats section, I see my comic's rank, but... what does that mean in the great scheme of things? Is there a top 100 list or something on DD? If you have a high rank, does your comic get moved to the top of searches..?

Sort of. If you've got a comic that's ranked #100 overall, then when someone browses comics by readership from most to least, then yours would appear further up than someone whose comic ranks #400, for example. However, people can choose not to view comics by readership, so in that case, you wouldn't have an advantage. You'd especially have an advantage, though, if someone browses by genre AND readership from most to least - if you're #100 overall, you'll probably be ranked even higher for your comic's genre. You might be #100 overall, #50 in stories, and #20 in fantasy, for example.

Ranking is also a cool way to see where your comic is at in terms of popularity. Ranks are determined by the number of favorites and page views your comic has, I believe.