SPY VWAP adds the VWAP indicator for SPY on your current chart, and shows the current SPY VWAP level converted to ES / MES value. It uses the last close price of SPY and ES / MES to calculate the level.
By adding the regular VWAP indicator to your MES / ES chart, you will clearly see the difference between the VWAP of SPY and the futures chart. This is helpful when trading as price may respect both VWAP levels.
This indicator should only be used on ES and MES futures chart. It will behave weirdly if used on different tickers and it is not supported in the current version.