Bank transfers are exactly what they sound like – a way to record a transfer from one of your bank accounts to another. There’s nothing special or magical about this process.
A common example of where this is used is when you pay the outstanding balance of your credit card at the end of the month. You’re simply sending money from your main account to your credit card.
You wouldn’t record this as an expense because you’ve already recorded all the transactions on your credit card as individual expenses, each with its own payment made on the card.