This article describes Sandbox from a user's perspective. Those looking to contribute to Sandbox development should visit the Sandbox project page
Sandbox is a library (and helper utility) to run programs in a "sandboxed" environment. This is used as a QA measure to try and prevent applications from modifying files they should not.
For example, in the Gentoo world we use it so we can build applications as root and make sure that the build system does not do crazy things outside of its build directory. Such as install files to the live root file system or modify config files on the fly.
All Gentoo installations come with Sandbox. Like all data, there is a possibility Sandbox can become corrupted or even uninstalled, which is very bad. If this is the case there are ways Sandbox can be recovered.
emerge --ask sys-apps/sandbox
There are multiple files used to configure Sandbox.
See /etc/sandbox.conf and /etc/sandbox.d/* configuration files for more information.