A Cookie is bits of data stored from the site onto the user’s computer. These bits of data are the presences the user has chosen. It is stored on the user’s device so that is the user was to go back onto the site the presences they chose last time would be remembered and applied.
There are some cookies that are known as tracking cookies that collects data about which parts of the site you interact with and for how long. This is useful for companies as it can show which parts of the website are most popular and which ones need improvement.