What is proof of relay in NKN?

Could you elaborate on the concept of "proof of relay" in the context of NKN? I'm curious to understand how it functions as a CORE component of the network's operations. Does it serve as a validation mechanism? How does it ensure the integrity and reliability of data transmission across the NKN ecosystem? What are the key factors that constitute a valid proof of relay, and how does it differ from other consensus mechanisms? Additionally, what role does proof of relay play in the scalability, decentralization, and security of NKN? I'd appreciate a concise yet thorough explanation of this fundamental concept.