The concept of reconciling accounts is typically not a favorite activity for entrepreneurs. BUT IT IS CRITICAL if you want to have an accurate set of financial statements in QBO.
First you will need to gather your bank statement. From QBO, you will click the Gear Icon > Reconcile.
After you select the correct account (ie checking, savings, credit card) you will enter the ending balance and the ending date then click continue. You will then compare your register in QBO to the bank statement. Everything that is matched gets checked. You would leave a transaction uncleared is it hasn't yet been posted by the bank. For example, if you wrote a rent check on the last week of the month but the vendor cashes the check during the first week of the month, then you will not reconcile that rent check until the following month.
Reconciling accounts is so important. I find that many small business owners don't do it or think they will do it and fall months behind. The good news is that is is a relative simple process after you get started you will be able to keep up like a pro.