A minimum daily balance, or minimum balance is the amount of money a financial institution will require you to have in your account to avoid monthly maintenance fees. Checking accounts almost always have a minimum balance requirement to avoid fees and savings accounts, including online savings account options, often have a minimum deposit required to open an account.
For example, if your minimum daily balance is $500, that means that if you have less than $500 in your account you could get charged a monthly service fee.
Though online banks tend to charge less fees in general, online checking accounts can still have minimum daily balances with fees attached. To combat this cost, you could look into an interest-bearing checking account with an internet bank.