What does 'dry' mean?

It means lacking flavor --- written in a "just the facts" style

 

It means there's a low concentration of moisture per unit volume in that particular system.

 

I agree that dry means, just the facts. For example, a comedian who is dry will say jokes like "a man walks into a bar, ouch!" or "I was driving home and turned into my driveway, but it was a dead end". It is pejorative, it usually implies the author is unaccomplished. A dry comedian is not very funny, a dry book is not terribly well written, etc, the authorship is poor. So you'll see reviews say things like "on the dry side (or a little dry) but has good coverage", the coverage is excusing the dryness.

I suppose it is a little like sophisticated. Sophisticated used to mean, convoluted, terse, but now it it seen as meaning advanced or intellectual. So "the treatment of differential forms is dry and sophisticated" could be a recommendation.