Cookies are small text files that are stored on your computer or mobile device by the websites you visit. The tasks of cookies are different. Cookies help a website to remember your settings, e.g. language, font size, username, and other settings so you don't have to change them again and again when you visit again.
Some cookies are necessary for the operation of the website, some are intended to improve the user experience, and some are also used to monitor visits to the website, which helps its manager to continuously improve the website and the user experience with it.