The bank allows you to save your coin, items and XP
Money you earn in game is “in your pocket” until you put it into the bank. Most transactions require you to have your money on you, for example using shops and buying claims.
Higher ranked members have the option to access these facilities out in the wild, but starting out you will need to access the bank from the Bank Building in Spawn (northwest bridge):
Interest is earned on Funds in your bank, when you are online.
/bank info will give you infor about your balances and any interest calculations. You can ignore exp interest as interest on exp is disabled and it will likely show as a negative amount.
With the right rank you can use /bank open to access the bank at any time.