What is key hash in Android?

Can you please explain to me what exactly is meant by "key hash" in the context of Android development? I've heard it mentioned in relation to integrating certain services, such as Facebook login, into Android apps, but I'm still a bit unclear on its significance and how it works. Could you break it down for me in a way that's easy to understand, including its purpose, how it's generated, and why it's necessary for these types of integrations? Thank you in advance for your help!