Bank institution allows to obtain extra cash in the form of a loan.
The maximum size of the loan and its monthly interest rate depends on the player's difficulty. Interest is taken from the account automatically every month.
Some assistants have the skills Banking and Interest Relief, that affect the amount of available credit (allow you to increase the credit available for the player or the size of the interest by changing interest rates).
If you have an assistant in each round when you can repay the loan (the player has the enough cash on account) It will remind the player of such possibility.