A cookie is a text file containing small amounts of information which is placed by a website within a computer or device through your web browser; the cookie is subsequently sent back to the same website by the browser.
Cookies are designed to assist your computer or any smart device to remember something the user has done within that website e.g. remembering that the user has logged in, or which buttons have been clicked.
There are specific cookies related to knowing a user is logged in or not. These are known as authentication cookies. Authentication cookies are important for the website to know what information to show you, or if it needs to ask you to log in.