A hashtag is a word or a phrase prefixed with the pound symbol (#). Hashtags provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it.
I am not a hashtag user, but it seems over-used, and sometimes confusing. For example, I had no idea what #TBT stood for. Or what about #ICYMI?
TBT = Throwback Thursday
ICYMI = In case you missed it