A stable coin is a token that, using some mechanism, is able to hold a stable value. The most prominent group trying to make this work using ethereum, I think, is http://makerdao.com/docs/.
A real-world warranty, which means you can't rely on it. You might own as many EthUSD tokens as you want but you can never be sure that someone will buy them from you at the 'fixed' price.
[1] - https://en.wikipedia.org/wiki/List_of_cryptocurrencies