Could anybody explain for the uninitiated?

It ensures that when you try to upload (or “put”) a new version of a file, the operation only succeeds if the file on the server still has the exact version (ETag) you specify. If someone else has updated the file in the meantime, your upload is blocked to prevent overwriting their changes.

This is especially useful in scenarios where multiple users or processes are working on the same data, as it helps maintain consistency and avoids accidental overwrites.

This is using the same mechanism as HTTP's `If-None-Match` header so it's easier to implement/learn