When a board is dry, most hands are no pair (not listed in the image) or one pair. There are very few two-pair hands, and usually no possible straights or flushes. In fact, so many hands are one pair, if you have two pair or better, you have an absolute monster.
On the other hand, when a board is wet, your two pair might be a middling hand, or it might be an underdog, depending on how much action there is. Every bet or raise that you see is less likely to be a bluff or weak hand and more likely to be a strong hand like a straight or a flush.
If our opponents fail to realize this point, we can capitalize. By check-raising on dry flops, our goal is to fold out hands that didn't hit a pair as well as hands that hit middle or bottom pair. If our opponent fails to realize that he only has top pair or better 10% of the time, then we are able to take advantage of him every time we check-raise as a bluff. On the other side of the coin, we do not want to check-raise wet flops with hands like two pair or a bluff. In my experience, people call far too light on wet flops, often with just a draw or top pair. We can take advantage of this by playing our very good hands, like trips, sets, straights, and flushes, fast and trying to see a cheap showdown with everything else.
