Bitshares is a DEX ( Decentralized Exchange) You own the private key to your account so you money is 100% safe as long as you keep your private key safe. BitUSD or other smart coins are digital assets thats value is pegged to other currencies or commodities like gold/silver.
That way you have a coin that is stable in value yet tradeable in a decentralized way.
RE: From BTS:USD to BTS:BitUSD What Does This Mean?