The US Dollar Index (DXY) measures the value of the dollar against a basket of major currencies (heavily weighted to the euro). It's a broad gauge of dollar strength, and because the dollar is the world's reserve currency, its moves ripple through nearly everything traders watch.

Why stocks care

A rising dollar tends to be a mild headwind for US stocks: it makes US exports pricier abroad and shrinks the dollar value of multinationals' overseas earnings, and it often coincides with tighter financial conditions (Fed policy). A falling dollar tends to be a tailwind. The relationship is loose and regime-dependent, but the inverse pull shows up often enough to matter (what moves SPY).

Dollar and commodities

Commodities are mostly priced in dollars, so a stronger dollar generally pressures commodity prices (it takes fewer dollars to buy the same barrel), and a weaker dollar lifts them. That flows into energy and materials stocks and into inflation expectations (PPI).

The dollar is the tide under everything priced in it. You can trade stocks without watching it — but you'll understand more of the "why" if you do.

Using it as context

You don't trade the DXY to trade SPY — you read it as background context, one input into the market's risk tone alongside rates and breadth (market breadth, the yield curve). A sharp dollar move is a reason to expect cross-asset volatility, not a standalone signal.