What is a transfer

You use the Transfer option when you're moving money between your own accounts within QBO.

So if you have a lot of cash and want to make a deposit in Checking, you would make a Transfer to show the money going out of your Cash on Hand Acct and into your Checking Acct.  (Be sure to use the date you actually go put the cash in the bank if your account is linked and you want the deposit to match.)