Cookies are small text files that a website stores on your device. They allow the site to recognise your browser on a return visit or during the same session, so it does not treat every page load as a first meeting.
A cookie itself is not a program and cannot read files on your device or look at other sites you have open. It carries a small amount of information — usually an identifier or a saved setting — that the site can read back on the next request. Related technologies such as local storage in your browser work in a similar way, and this policy covers them too.
Some cookies are necessary for the site to function, others are used for measurement or to hold a preference you have chosen. Session cookies disappear when you close the browser; persistent cookies stay on your device until they expire or you delete them.