Cookies are small files stored by websites on your computer that allow them to track you around the internet. Most cookies are innocent and harmless.
By default, CCleaner removes all cookies and so this means you will need to log in to these sites again.
If you want to stay logged in to these sites, there's an easy workaround.
Go to Options > Cookies and drag the cookie for the site concerned (e.g. mail.google.com for Gmail) to the right hand panel.
Now you can clean your browser, and you will stay logged in to your sites.