What's an "Endpoint"? And how does one enter the "Encryption Key"?
Good questions.
An 'endpoint' is the URL of a cloud server running the Bitcoin Backup server software (really just a PHP or Python script that accepts the upload and does a few other things to store it securely and keep track of it. The idea is that a bunch of people would be running endpoints.
The encryption key is a password you will provide that will allow the program to encrypt the wallet file(s) using AES256 encryption. It's never transmitted over the wire but only used to encrypt the wallet file (which is stored in a .ZIP) before it's uploaded to the server.