That means you write it, send it, store it, operate on it, etc. with little or no modification.
The author says "converting between the two is trivial" which may be true, but the developer overhead is less trivial. And it will always be JSON in the client - JS doesn't support YAML objects.
Is that really the selling point? I interact with numerous services that speak JSON...and most of them aren't written in JS. YAML and JSON both have to be parsed to be used, even by JS. Otherwise it's just a string.
To me the real selling point in JSON is the dead-simplicity for humans to read and CHANGE. YAML is human-READABLE, but frankly I often screw up changing it because the formatting is a little too magical and I'm a little too unfamiliar. JSON is downright picky and obnoxious...but that makes it really easy to make a change. Screw up the quotes? It'll complain about the quote. Dangling comma? It will complain. Did I cut-and-paste from an HTML display that screwed up my whitespace by compressing everything down to a single space? Nothing cares.
I remember mongodb started with json, but switched to in-house bson pretty early because of the json limitations.