It is a piece of US coinage worth ten cents minted from 1916 to 1945, and is properly called the Liberty or Winged capped Liberty. It is a prized coin by collectors as one of the most beautiful US dimes minted.
The image looks very much like a picture of the Greek god Mercury who used his winged helmet to fly through the heavens. Because of that the coin was mistakenly called a "Mercury" dime almost from its first day in circulation, and the name has stuck for nearly a century.
All Mercury dimes were minted in an alloy of 90% silver and 10% copper, so today they're worth much more than 10ยข just for their metal alone. In addition, many older dates (1930s and before) can be worth even more to a collector.