A stablecoin is a new class of cryptocurrencies that attempts to offer price stability and are backed by a reserve asset. 

Advantages of asset backed cryptocurrencies are that coins are stabilized by assets that fluctuate outside of the cryptocurrency space, that is, the underlying asset is not correlated, reducing financial risk. Bitcoin and altcoins are highly correlated, so that cryptocurrency holders cannot escape widespread price falls without exiting the market or taking refuge in asset backed stablecoins. Furthermore, such coins, assuming they are managed in good faith, and have a mechanism for redeeming the asset(s) backing them, are unlikely to drop below the value of the underlying physical asset, due to arbitrage.
Backed stablecoins are subject to the same volatility and risk associated with the backing asset. If the backed stablecoin is backed in a decentralized manner, then they are relatively safe from predation, but if there is a central vault, they may be robbed, or suffer loss of confidence.