The storage map is an overview how storage devices are filling up with various data types, such us: audios, movies, photos, applications, backups, …
The storage map is part of the System Profiler, and you can find it from Apple menu if you select About This Mac. Click here on More Info… button. Select Storage tab on the top-left part of the next window. You will see here devices connected to your computer:
It shows how much space is free out of capacity of the devices also how much space being used for specific file types on each devices via graphical representation.
Please note: this is a quick overview only!