In computing terms a cookie refers to a small text file stored on a user’s computer by a web site the user might be visiting. Information stored in these cookies are used by web sites to store information related to the user for example the email address, user name or anything else the web site might need to identify the user in the future.
Web sites implement this to improve user experience and personalisation. That is how the web sites know who you are when you visit them a second time round. Clearing your cookies in your browser will delete all this information and the web site will not recognise the user.
Cookies can be disabled in most browsers but this can prevent some web sites from functioning properly.