This screen lists all the storage locations which have been found on the device. On many modern devices, this will generally only consist of the Internal Storage, but can also include SD cards and connected USB storage devices. If a storage device isn't found, you can add it by selecting "Choose Folders..." from the main menu.
Prior to scanning, the total size, free space, and used space - as reported by the file system - is shown. In some cases, particularly with detachable storage devices on newer devices, the sizes may not be available until after scanning due to permission being required
Click the scan button to scan or rescan the device. If possible, it will immediately begin scanning. For some storage devices, permission may be required, which will be requested prior to scanning.
Once scanned, the total space used by readable files will be shown in blue.
It will also show the estimated overhead due to the filesystem. This includes the folders themselves and extra space used by files because of the minimum allocation sizes. Usually on Internal Storage, this will be negligible, but can be larger on external storage devices and/or where there are a significant number of very small files.
The known used space minus the readable files is shown as inaccessible. On the Internal Storage, this will largely be the space taken up by apps and their data files.
Tap on the pie chart to open the folder and view the files and sub-folders in detail.