If the account you're importing into is completely fresh — no balance, no transactions yet — you'll be offered the chance to set its opening or closing balance so it lines up exactly with your bank statement, either by typing the number in or by pointing to a running-balance column already in your file. Just know this option only ever shows up the very first time you import into a given account.
The balance is worked out from the rows you actually select on the review screen, so deselecting rows changes the result — and the wizard states the opening-balance assumption in plain words before you import. If you imported by mistake and removed every transaction, an account with no transactions lets you edit its initial balance again from the account form.