What is PSP fee?

Excuse me, could you please clarify what exactly is meant by the term "PSP fee"? I'm a bit unfamiliar with the terminology and would appreciate it if you could break it down for me. Is it a specific type of fee associated with a payment service provider, perhaps? And if so, what kind of services or transactions does it typically cover? Additionally, is the fee charged by the PSP a fixed amount or does it vary based on certain factors, such as the transaction size or the payment method used?