sUSD is Synthetix native stablecoin which you can obtain by staking SNX - native crypto on Synthetix DeFi platform. Its price flucutuate around USD (usually sligtly lower). The whole process is designed as typical for DeFi (minting sUSD for staking SNX) and works as an lend - you simply borrow the sUSD for collateral (SNX). The main problem with sUSD is its popularity, it lacks higher adoption and is used solely at Synthetix Exchange. I would rather borrow USDT and have higher variability. The whole Synthetix project was just hyped around DeFi boom.