The HTTP ETag (entity tag) response header is an identifier for a specific version of a resource.
It lets caches be more efficient and save bandwidth, as a web server does not need to resend a full response if the content has not changed.
Additionally, ETags help to prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions")....the If-Match headers, you can detect mid-air edit collisions (conflicts)...W3C Note: Editing the Web – Detecting the Lost Update Problem Using...