What is an ETag and how does it work?

The Entity Tag or ETag is the HTTP Header which is used for the validation of web cache and for the conditional requests from browsers for resources. They can be also referred to as identification elements that have been tagged to the user’s browser. The user is allowed to remove HTTP cookies and they store the information which is same along with backup if in case there is a need to reconstitute the data of deleted cookies.

Working of the ETag is explained below:
I. Firstly, Server receives a normal HTTP request for a particular source, for example, ABC.
II. Server side will now prepare the response. The server-side logic wants the browser to cache ABC locally. All the browsers cache the resources always by default.
III. The server will include the header ETag with its value.
IV. The response will now be sent from the server to the header, content of ABC in the body with status code 200.
V. Then, the same browser will make another request for the same resource ABC.

Thanks @Fairy Kumar For your valuable answer.