Cookies are basically small files, they ask you if you will allow permission for them to be placed on your hard drive. Once agreed a tiny file is added.
They help to draw up analytical data looking at web traffic, once agreed on a particular site it remembers you.
When you allow cookies, the web applications will respond to you individually. Which can offer you personally suited information.
So in a nutshell, by using cookies, they will gather and remember your preferences which means you can receive tailored information as the 'Clever Cookies' remember your likes and dislikes.