An opening balance is the starting point for the account. It summarizes all past transactions in your up to the opening balance date. To reconcile an account accurately, the beginning balance in QuickBooks Online has to match the beginning balance on your bank statement.
When your QuickBooks Online beginning balance doesn't match the bank statement's, it's called a beginning balance discrepancy. Here's an article on how to fix an opening balance in Quickbooks Online.
Open Journal Entry
Correct Journal Date
Name Journal no. (ex: "Opening Balance" following the account #)
Check the box "Is Adjusting Journal Entry?"
Select the Account
Fill in Debit when there's money in the account (or Credit when it's negative and no money)
Select the Opening Balance account fill in the Debit or Credit
Save and close
Here's a video tutorial on how to fix an opening balance in QBO.
Here's a Youtube Channel of more recently updated video tutorials for Quickbooks Online.