It is a good looking graphical overview of how much space is in use, and of how much is free or available. For example, my phone has 29 Gigabytes of storage (likely it's 30 GB total, with 1 GB going to the OS). 16 GB is in use. You can see the breakdown below. The Details section shows specifics, like how much space is taken by Photos, Email+contacts, Office Documents, Ringtones etc.
You can also go into Details and clear Temporary files manually, although the OS does this periodically for you theoretically. I had 630 Megabytes of Temp files so I cleared them manually.[I did not screen shot this]. Note that some level of Temp files are normal and cannot be cleared.