Simple definition
An ETag is an HTTP response validator that lets clients check whether a resource has changed before downloading it again.
Purpose and use
An ETag identifies an HTTP representation version for validation or concurrent-update control.
How does it work?
Its value is opaque and may come from a hash or another mechanism. The W/ prefix marks a weak validator with specific comparison rules.
Example and verification
A client can use If-None-Match to revalidate cached content or If-Match to prevent updating a version that has become stale.
Points to watch
An ETag is not an authenticity signature and does not prove that separate resources are identical. Weak validators are unsuitable for some operations.
In short
Interpret the validator in the context of the representation and HTTP condition to which it applies.