This is a simple rule, and a good one, and is often illustrated like so: don’t write ‘he said angrily’, write ‘he shouted’. Don’t write ‘the dark blue ball’, write ‘the cerulean ball’ (or I am getting my shades of blues all mixed up?).
It’s mainly aimed at new authors who do have the tendency to go a little nuts with stringing on the descriptors. It improves prose because too many descriptors can disrupt or weigh down the flow and can, strangely, make a scene vague despite how many adjectives are being thrown at the reader.
But this is another rule that can be taken too far. I was once at the point where I was scared to put in any descriptors at all, and was accused by one editor of having a too ‘simplistic’ writing style (and here I thought ‘clear and simple’ was a goal, not a negative). I had to start adding descriptors back in.
On the other hand, using words like ‘cerulean’ and other rarities that you have to capture from a thesaurus is not a brilliant idea either, mainly because a thesaurus can’t explain nuance and the in-the-real-world subtle connotations of the differences between, say, ‘cerulean’ and ‘indigo’: if you don’t really know a word well enough, you might not want to use it.
The rule shouldn’t be about what you shouldn’t be doing. It should be stated like this: be precise. Say exactly what you mean, as specifically as you can. Actively choose strong verbs and nouns rather than settling for the first word that comes to mind. And when you do use an adjective or adverb (we are actually allowed to use them or we wouldn’t have invented them), choose that just as consciously.
It’s also another good rule to ignore on the first draft, worry about on the last (get the story right first).