What does Clear Cookies and Data mean for Safari on iPhone?

It's under Settings>Safari
What does it do? I just don't want to lose my bookmarks.

Cookies are data automatically sent between the device and the browser. For example, a cookie could be a saved password for a website.

Cookies means all your login data are saved in it. Data here refers to the browsing history, forms, temporary files etc. There is nothing wrong in clearing them. Infact clearing them speeds up your browser. Bookmarks can be deleted or edited separately.