I keep on seeing this, what does it mean?

[Kendall Spears]

On facebook, twitter and everything I keep on seeing people use this symbol "#" I might be slow or late whatever but its been on my mind so much. Can someone tell me what it means!! They use it before something is said

[supensa]

It's a Twitter hashtag. When used before a word, like "#thisisahashtag", it's basically the unique "subject" of the tweet, so that you can remember it and come back to it later, or for other people to search for it.