Banking online, any accounts you have in that bank or associated through that bank are available to see and make transactions, e.g. write checks, deposit savings, pay credit cards issued by the bank. Depending on the bank's setup, you may even be able transfer money instantaneously to someone else's account if necessary.
It automatically does the math for you..write a check and it deducts it from what's available in your account. Same with deposits..added to the acct. You'll always know just what you have to work with.