What are cookies?
In the context of the Web, cookies are little bits of information that the particular Web site that you are on at the time sends to your browser.
Why are cookies important?
Your browser stores and saves these little bits of information on your computer (usually in your temporary Internet files folder). The next time you visit that Web site, this information is retrieved so your visit is more customized with less load time effort for your browser. Cookies are actually very helpful because they help Web sites make your visits more personalized.
Should I clean out my cookies?
You don't want to completely disable cookies because your Web search experience will definitely suffer, but you should clean them out every once in a while to optimize your Web browsing experience. A detailed tutorial on how to do this for any Web browser can be found here: How to Delete Cookies in Popular Web Browsers.