The name of the internal storage folder in an Android device that holds the user's apps and media. The sdcard-zero naming is confusing, because people think of an SD card as an external module that is plugged in rather than built in. Most Android devices also support additional storage via "external" SD cards. See SD card
The SD Card Hierarchy
The ES File Explorer utility software in this Android phone shows the device's primary folder hierarchy. There are numerous subfolders within each of them.