Cookies are usually small fragments of data transferred to save in your browser in order to identificate yourself. That includes account authorization and session information.
They are sent to Web servers to process certain functions.
They are usually necessary to let the Website function correctly.
3. Are cookies secure?
Cookies are usually secure as they have properties. Websites may limit the domains able to read them (eg. to only themselves) and force http protocol. Each cookie has also an expiration date, so after this date, your browser no longer sends it to the server.
4. Can I clear the cookies?
Surely! You can clear them at any time in the browser's settings and it will make you anonymous completly until using authorization functions again.