Media Cleaner is a tool that detect unused and useless files from your WordPress. It cleans clean your Media Library and your Uploads Directory.
You can choose to check your Media Library or your Uploads Directory. Then the plugin will check if the file (or files, in the case of a media) is used anywhere in your WordPress install depending on the options you have chosen. Here are a few examples of what can be checked:
Basically, by playing with the options, you can discover a lot about how your system is used and how much (in term of weight) of images became useless.
The files (or media) is then added to the Media Cleaner Dashboard. From there, you can choose to move then in the trash. The next step is to test your website, maybe for a few days or more; if nothing is missing, if all is working well, it means none of those images were actually in use. Then you can decide to trash them completely.