What is protocol fee?

Excuse me, could you kindly elaborate on the concept of "protocol fee" in the realm of cryptocurrency and finance? I'm somewhat unfamiliar with the specific term and its implications. Could you please explain in simple terms what it represents, how it is calculated, and its significance within the broader cryptocurrency ecosystem? Additionally, are there any particular protocols that commonly impose such fees, and if so, what are some examples? Thank you in advance for your clarification.