A currency board exists when a country’s central bank commits to back its monetary base- its money supply- entirely with foreign reserves at all times. This means that a unit of domestic currency cannot be introduced into the economy without an additional unit of foreign exchange reserves being obtained first.
Currency board is a monetary regime in which the domestic currency is backed 100% by a foreign currency (say, dollars) and in which the note-issuing authority, whether the central bank or the government, establishes a fixed exchange rate to this foreign currency and stands ready to exchange domestic currency at this rate whenever the public requests it.