What is the significance of a hashtag?

[persoND]

So I've heard people say "Hashtag this" and "Hashtag that" and am oblivious to what this means. I live a pretty reserved life in terms of the internet (no facebook, twitter, etc) and understand that if I were a part of social networking sites that I most likely have heard about these things, so please: no comments saying that I should get one or that I am a moron because I don't understand what a hashtag is. I am asking a reasonable question and deserve a respectable answer.

Thanks!