What is a Sandbox Directories in iOS?

So many iOS hackers and developers out there always talk of sandbox, but what is a sandbox? Read below i'll try to explain what's a sandbox directories in iOS.
The App Sandbox

For security reasons, iOS places each app (including its preferences and data) in a sandbox at install time. A sandbox is a set of fine-grained controls that limit the app’s access to files, preferences, network resources, hardware, and so on. As part of the sandboxing process, the system installs each app in its own sandbox directory, which acts as the home for the app and its data.

To help apps organize their data, each sandbox directory contains several well-known subdirectories for placing files. Figure A-1 shows the basic layout of a sandbox directory. For detailed information about the sandbox directory and what belongs in each of its subdirectories.

Important The purpose of a sandbox is to limit the damage that a compromised app can cause to the system. Sandboxes do not prevent attacks from happening to a particular app and it is still your responsibility to code defensively to prevent attacks. For example, if your app does not validate user input and there is an exploitable buffer overflow in your input-handling code, an attacker could still hijack your app or cause it to crash. The sandbox only prevents the hijacked app from affecting other apps and other parts of the system. – iPhone Programming Guide, Seite 130
For security reasons, iOS places each app (including all preferences and data) placed in a sandbox at install time, so it's called sandbox. The sandbox has controlled and limited access to the file system settings, network resources, hardware and so on. All apps are installed in a sandbox, which is regarded as the home directory for each app and its data.
Note: For more details about the sandbox directories and what belongs in that directory, you can read in the File System Programming Guide.
Note: The purpose of a sandbox is to protect the system from compromised apps so they won't damage the system. This is exactly why the iOS hacker wanna break out of the sandbox, so they can exploit to the system.