The Payment tab is where you decide how supporters can pay — for example by Direct Debit or by card — and how those choices appear on your form.
This is what the Payment tab looks like in the editor:

And this is what the settings above produce on the form your supporters see:

There are three things you can set up here:
The Heading is the short line of text shown above the payment buttons. If you leave it blank, no heading is shown.
For example, entering "Checkout" here:

…shows "Checkout" above the payment buttons, as in the form example at the top of this page.
Each row in the Payment options table becomes one payment button on your form. In this example there are two rows, so the form shows two buttons: