It scans your system for duplicate files and folders and offers you a list of files you can delete.
Those lists are usually huge. You usually have two or three copies of the same photos, you download the same file twice, you receive presentations,... and all of them are there in your HD.
The longer your HD is, the more rubbish you will have in there.
So we recommend you to download and run DupScout, it will set a lot of MBs free and your computer will run better.