What is KakaoPay in English?

Could you please explain to me what KakaoPay is, in simple English terms? I've heard about it but I'm not quite sure what it's all about. Is it a payment service? An app? Or maybe something else entirely? Could you also tell me how it works and what makes it unique compared to other payment methods? I'm quite curious about this and would appreciate it if you could provide a clear and concise explanation.