A reserve currency is one that central banks and major institutions around the world hold in significant quantities as part of their foreign-exchange reserves, used to settle international trade and stabilize their own currency. The US dollar is by far the dominant global reserve currency, with the euro, yen, and a handful of others playing smaller roles.
Reserve-currency status gives a country's currency structurally deep liquidity and outsized influence on global capital flows, which is part of why USD pairs dominate forex trading volume.
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How to Check the Way a Currency Is Managed
The regime behind a currency is published, not inferred. The four primary sources that say how a rate is governed, what each one can and cannot tell you, and how to keep the answer from going stale.