A split payment is where the customer pays their bill using 2 or more different types of payment methods. For example, paying half the bill in cash, and half using a credit card. This is not the same as paying in installments, where a bill is paid in parts over a period of time.
This concept is easy to understand in physical stores and restaurants. Someone might pay for an item using cash and card, or split the bill at a restaurant between multiple people. These are both examples of a split payment.
With online commerce, the process is a bit different since most platforms still don’t accept split payments. You can typically only pay the entire amount using one credit card for example. The common exception is with gift cards. You might have a gift card for $50, but to purchase something for $100, the eCommerce platform allows you to pay the remaining amount with a different payment method. Very few websites will allow you to pay using 2 credit cards, or 1 credit card and your digital wallet.