Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.
This particular exercise requires that you use the raise statement to "throw" a ValueError for when an account is or is not open, or the withdrawal/deposit amounts are incorrect. The tests will only pass if you both raise the exception and include a message with it.
To raise a ValueError with a message, write the message as an argument to the exception type:
# account is not open
raise ValueError('account not open')
# account is already open
raise ValueError('account already open')
# incorrect withdrawal/deposit amount
raise ValueError('amount must be greater than 0')
# withdrawal is too big
raise ValueError('amount must be less than balance')