Good question. You should always understand software that you are using.
It is safe, good, and it works and its free. I use it and so do many members.
After you download, you can leave it on the default settings. Use it once a week or so to clean all files that are not needed.
Any questions about settings, etc. just ask. As I said I use it, too.
The only problem that I had was that I did not put cookies (from sites that I log into, personal setting, etc) into the save section. The only difficulty was that I had to reset a few sites, but then preserved them by moving the cookies.