It's a token which is backed by approximately $1 worth of Steem. You can send it to other Steem users with no transaction fees, to buy something or just to sell it for Bitcoin or normal money.
It is supposed to be worth about $1, but it can be worth more or less than that (such as now it is worth about $1.45).
Thanks for that reply demotruk, I didn't realise that I could send it to other Steemers.