How does proof of reserves work?

Could you please explain how proof of reserves functions in the world of cryptocurrency? I'm curious to understand the process behind it and how it ensures that exchanges and custodians are holding the funds they claim to have on behalf of their clients. Specifically, how does the verification process take place, and what kind of evidence is required to prove that the reserves are indeed genuine and sufficient? Also, are there any potential limitations or risks associated with relying solely on proof of reserves as a measure of trustworthiness?