By the time you’ve finished writing it, you’ve probably figured out what it’s about. Not what happens in it, but what it’s about. What it means. What it’s for.
On your second draft, you buttress the stuff that makes that theme work. You chip away at anything that detracts or distracts from it.
Also, you try and read it for the first time, pretending you are a reader. You fix anything that would irritate you-as-a-reader. You write the bit you were sure you could skip as a writer, but you-as-a-reader wants to see. You trim the bits where you-as-a-reader found yourself wanting to skip, or wishing the author would just get on with it.
You make it better.
Seems like very good advice.