Definition of dry
When we talk about dry, we usually mean a dry flop (a dry flop as opposed to a wet flop). By dry, we mean that the flop (or board) does not offer an obvious draw. The classic example of a "dry board" is A-7-2 rainbow (with three different suits, say hearts, clubs and spades, thus offering no color draw). Doubled flops are also dry flops (e.g. 2-2-6, with no color draw).
How to play a dry flop?
Dry flops should be played differently from drawy flops. This is especially true if you have a very strong hand. If you have AA on the flop we suggested above (A-7-2 with no flush draw), you are probably better off playing passively. Your opponent will have almost no reason to call your bet and you may lose. On the other hand, if you have a bluff on A-7-2, you may have more success since your opponent will not call with a draw and unless he has top pair or better (a minority of the time), he will be forced to fold. And finally, as a general rule, there will be fewer bluffs and semi-bluffs on this type of board. When your opponent shows strength, unless there are notes to the contrary, give him credit.