If you're confused with what it actually is:
Well, your double posting which is posting more than once again and again, without anyone else's response in beween. Qouting is when you quote what someone says, to give a specific response to, multi quoting is when you want to quote several responses at once. for example, if around 4 people have welcomed, you instead of quoting and replying, quoting and replying and doing this again and again individually for all of them, you can do it all at once.
To multiquote, click on the " at the bottom of someone's post, then continue clicking on all the people yiu want to respond to. Then scroll down to the bottom of the page and select 'reply'
Hope that helped