The Bank is a feature that allows you to store Coins and Items. You cannot spend or Donate Coins while they are in the Bank, and you cannot use Items while they are in the Bank. Coin amounts slowly increase while in the bank (1% every other day), and having Items in the Bank can allow you to have more than 20 Items, which is the normal max amount.
<C++ jargon>Unfortunately, Francis created the interest system using int variables instead of passing floats to ints</C++ jargon>, so you have to have at least 100 coins in the Bank before you get ANY interest...