I assume you are speaking of dry muscle gains. It is the lack of bloat and muscle and skin tissue saturated with water and usually a higher bf%. Water retention makes muscle seem larger and fuller than drier tissue which is harder and more defined. There is more actual muscle tissue per volume vs the same size of a "wet" muscle gain. The terms "shredded" or "ripped" are common terms used to describe the physique of someone with a dry muscle composition. There is much to be said about the relationship of the 2 in the overall size gaining process but I haven't the time to go there. Besides you only wanted to know what "dry" meant in the bb world. Someone else can take it from here if they wish..