When DXY (the dollar index) goes down, share prices go up as a result (after excluding all other price signals). If dollars are worth less, then it takes more dollars to buy a share of a company that owns the same assets and has the same outlook. It’s an inverse relationship similar to bond yields and bond values.

You can use TradingView to look at a chart of SPX/DXY to see a chart of SPX adjusted with respect to DXY, IIRC you just enter ‘SPY/DXY’ into the ticker symbol field and you get a dollar-adjusted SPX chart.