The Cookies are small items of data that consists of name and value pair. Cookies are stored on your computer so that it can be accessed by your web browser.

A web browser and server communicate through HTTP which is stateless protocol. Stateless protocol treats each request independently, so server does not keep data after sending it to browser. With cookies such data can be fetched directly from stored cookie file instead of communicating with server.

For example when user visits web page, user name can be stored in cookie. Now when next time user visits the page cookie belonging to the page is added to the request. This way server gets necessary data "remembered" by cookie.