By which he means, description is nothing without relevance to the story.
Like anything you wish to learn, the best way to do it is to break it down into smaller, component parts that are easier to digest and understand. We do this with writing, we break the novel down into baser elements – plot, character, dialogue, narrative, description, action, tension, exposition. We submit fractions of our prose to crit groups, to agents and editors – we polish and polish until our plots are complex, our characters fully-formed, our dialogue convincing, our narrative fluent, and our description evocative – and yet, something's missing.
We must master these elements and when we are fluent in them, dismiss them at will, using only those parts that evoke, reveal and describe the story we wish to tell.
Craft is just the long road to Art. It's ok to tell rather than show sometimes. It's ok to summarise. A general description can have more resonance than six pages of artfully worked yet irrelevant detail. The tools of our craft should be our servants, not our masters.
The rules should set you free.