A cookie is a small text file that is stored in your browser when you visit almost any web page. Its usefulness is that the website is able to remember your visit when you return to browse that page. Cookies usually store technical information, personal preferences, content customization, usage statistics, links to social networks, access to user accounts, etc. The purpose of the cookie is to adapt the content of the website to your profile and needs; without cookies, the services offered by any page would be significantly diminished.
If you want to consult more information about what cookies are, what they store, how to delete them, deactivate them, etc., you can find information further down on this same page.