What is US Dollar (USD)?
The US dollar is the most-traded currency, involved in ~88% of all FX transactions. Roughly 60% of global foreign exchange reserves are held in dollars. The US Dollar Index (DXY) measures USD against a basket of six developed-market currencies (EUR, JPY, GBP, CAD, SEK, CHF). The Fed sets US monetary policy that drives dollar value. Other central banks intervene in their currencies vs. USD, not the other way around.
Why it matters for investors
Dollar strength affects everything: emerging market debt servicing (most EM dollar-denominated debt becomes harder to repay), commodity prices (oil/gold inversely correlated with USD), US multinational earnings (stronger dollar reduces overseas revenue when translated), and global trade. The "exorbitant privilege" allows the US to run persistent current account deficits.