Your question is a little unclear because this Google Books link goes to a section of Emily Post's Manners in a Digital World: Living Well Online that puts making appropriate online posting choices in perspective by comparing it to dinner-table conversation, but this not the definition of etiquette, with of off-line or online etiquette. I'll suppose then that you really need is an explanation of the "dinner-table rule."
The dinner-table rule is very simple. It supposes an agreement (that may or may not exist) of the dinner table as "the most civil place in our lives" where conversation must not humiliate of jar and where adverse reactions to our missteps are inescapably evident because of the closely proximal nature of meal sharing. It states that if you can imagine an online post might cause embarrassment to you or anyone or any kind of adverse reaction to you, then do not post it: it is inappropriate since the sense of online anonymity and superiority to censure is an illusion at best and a delusion more often than not.