What are cookies?
They are small text files, which can be delivered to your browser from websites. Browser then saves them on your hard drive. Browser then sends them back to this website on the next visit. These files don't take up almost any space, their size is usually only a few kilobytes. You can find more information about cookies for example on Wikipedia here
What are cookies for?
Cookies can be thought of as a website memory, which, for example, can be used to recognize specific user.