A funding source is where the money for one transaction actually came from. Usually that is a single account, and you never think about it.

It matters when a payment came from more than one place — part cash, part bank, part on credit you already hold. A transaction can name several sources with an amount against each, so the record matches what really happened rather than being simplified into one account.

Getting this right is what keeps individual account balances true. A payment split three ways but recorded against one account leaves the other two quietly wrong.