Cookies are small text files that a website stores on your device and reads back on later visits. A cookie is written by the site you are visiting, is stored by your browser, and is sent back to that site when a page is requested again. It usually contains an identifier and a small amount of associated data, not the content of the pages themselves.
They are used to keep a site working between page loads, to remember preferences, and to measure how pages are used. A cookie that maintains a form session lets the contact form on this site know that an entry was started in the same visit. A cookie that remembers a decision about analytics saves you from answering the same question on every page.
Similar technologies, such as local storage and session storage, serve comparable purposes and are covered by this policy in the same way. Where this page says "cookies", it also means those technologies unless the difference matters to you — for example, local storage is not sent back to the server with every request the way a cookie is.