"Account balance" refers to the amount of money in a particular account, after considering both deposits and withdrawals. It shows how much money you have in that account at a specific point in time. Full definition
It inherently prevents any one from checking account balances of others. (thecollegeinvestor.com)
Of course, this method only works if there is enough money in the other account to bring the checking account balance out of the negative numbers. (discoverdebtfreedom.com)
Actual costs will vary based on account balance and investment strategy. (fidelity.com)