That’s easiest part (because I know the answer 
)
It is added to outgoing links and is suggestion to search engine bots not using and following those links. It is a try not to give ”search result points” to third party sites.
I don’t use it, because most of links from my forum deserves guidance to search engines, but situation is or may be different if users are linking to low valuable resources.
There is no right or wrong answer here.