To annoyingly "tag" objects, emotions or feelings by adding a "#" (hashmark) and then the thing wanting to be said.

what people say to people who are noobs to the internet

Used when expressing humour or sarcasm, usually when playing on an online multiplayer game (e.g. Minecraft)

twitter vocab.
if you understand the concept of hashtags, you'll understand "#hashtag."
it's basically a wild card for hashtags and leaves it open for interpretation for the reader.
it can even fill a void when no words can properly describe the extreme extent of the tweet (situation) perfectly. or use when you're having a hard time coming up with a clever hashtag.
when you can relate to the situation and/or picture it happening.

basically if you ever catch yourself saying "i dont know what to hashtag" use #hashtag