Unlike the previous ETFs, the ProShares Short S&P500 fund is a way to bet against the S&P 500 index. An investor might seek to profit by betting against an investment, as some investors did during the bear market of 2022. It can also be used as a hedge if you think the market might be a little overvalued, but you aren't exactly ready to start selling your stocks.
The ProShares Short S&P500 aims to provide a return exactly the inverse of the S&P 500 index's daily return. So, if the S&P 500 is up 1% on a given day, the ProShares fund will be down 1% (before fees, which total 0.89% on an annualized basis).