Mints are used to make coins. The coins created by the mints can be used as a form of currency in exchanging items with villagers, like the Emerald.
It requires a redstone pulse to press, and each press produces one coin. Clock circuits are recommended for pressing Electrum or Platinum.
When pressing large quantities of bars into coins, a Mint Storage can be placed touching the Mint to allow the Mint to auto-load bars from the Mint Storage when it's empty and a current is applied to it.