This is the first cryptocurrency issued by the Winklevoss twins' company, Gemini. A stablecoin is a cryptocurrency in which every coin, or token, is backed by and "pegged" to a stable asset. In this case, the Gemini Dollar is pegged to the US dollar. The Winklevoss twins are launching a stablecoin, along with dozens of other companies who have launched similar products in 2018, in order to increase the usage of cryptocurrencies as a medium of exchange.
Currently, most people use cryptocurrency to store value, similar to the way that people store value in gold. Bitcoin is mostly used for this store of value purpose. With the introduction of stablecoins, people should theoretically be more willing to use the cryptocurrency in real time as a medium of exchange for payments and other instant value exchanges. This is due to the reduced volatility of the coins. It remains to be seen whether these stablecoins are necessary in the long run as well as what their advantages are versus the US dollar itself.