Some cards (Switch before it disappeared) had an issue number, you should only need to fill in that field if your card has an issue number (as far as I can tell, the vast majority dont).
Issue number can often be found on the bottom-right on the front of most cards (most include it even if they don't use it when purchasing goods, for other purposes). However if you have say a Visa Debit-based card you shouldn't need to enter it.