ETag
In plain terms
A header identifying a specific version of a resource.
Going deeper
An ETag is a fingerprint a server assigns to a resource version, letting clients cache and revalidate efficiently. It enables conditional requests.
Under the hood
Clients send If-None-Match; a matching ETag returns 304 Not Modified, saving bandwidth.
Examples
HTTP caching, conditional requests, 304 responses, cache validation.
Think of it like...
Like a version stamp letting you ask 'has this changed since I last saw it?'