What is a fee charge?

Excuse me, could you please clarify what exactly a fee charge is? I'm trying to understand the concept better in the context of cryptocurrency and finance. Is it a fixed amount that needs to be paid for a specific transaction or service? Or does it vary depending on the type of transaction and the amount involved? Additionally, how does the fee charge get calculated, and what factors influence its amount? It would be greatly appreciated if you could provide a detailed explanation of the concept.