What is an ETag?
In one sentence: A fingerprint of a file the server sends so a browser can ask later whether that exact copy has changed.
A little more
An ETag is a short fingerprint of a file. When your browser already has a copy, it can send that fingerprint in an If-None-Match header and ask whether the file has changed.
If the fingerprint still matches, the server answers 304 Not Modified: keep your bytes, no new download. If not, you get a fresh 200 with the new file.
It's a bit like… Asking a shop 'is this still the same edition of the book I bought last week?' and hearing yes or no.