The ETag header presence is important for this header more than its value, which is not supposed to be read by the browser. However, its value is often W/<content length>-<last modified>, where content length is the size of the resource and last modified is its last modified timestamp. This is mainly because it is easy to generate and stateless for the server, but it can be anything, including a random string.
This header value can be used as a strong validator. The presence of W/ at the beginning marks it as a weak validator, which means multiple resources can have the same value.
The value is used with other headers as an identifier, for instance, Other-Header: <etag>.
Used with If-None-Match, the header takes a list of Etag values, ...
