The US Dollar Index measures the dollar's value against a fixed basket of six major currencies, heavily weighted toward the euro (which alone makes up well over half the index). It's the most commonly cited shorthand for overall dollar strength in financial media.
Because the euro dominates the index so heavily, the DXY can behave more like an inverse EUR/USD chart than a true broad measure of the dollar. A scenario where the dollar is genuinely strengthening against, say, the Australian and New Zealand dollars while staying roughly flat against the euro can show up as a flat DXY — even though real dollar strength is happening elsewhere. That's exactly the kind of nuance a full 8-currency strength view is built to catch, that a single index can mask.