what is registry? how does it work? what do issues mean? should i fix it?

Here is the part of the documentation that deals with registry cleaning

http://docs.piriform.com/ccleaner/using-cc...gistry-cleaning

May I suggest that you hold off on the registry cleaning (known as issues in the ccleaner program) until you become a bit more experienced.

Of course using the ordinary part of the cleaner should be fine

Welcome to the forum