The SHSH key is an informational portion that Apple uses to create a digital signature on all of its iOS devices, including the iPad, iPhone, iPod touch, and Apple TV. This information is used to manage the iOS versions that are compatible with the operating system. It usually allows the installation of the most recent firmware version.
This is precisely how jailbreak works. It allows you to download applications from other stores such as Cydia. The jailbreak process uses to alter this digital signature in order to install software not accepted by Apple's platform. Tiny Umbrella modifies the digital signature so it is possible to run new iOS versions without any problems.
This application protects the original digital signature prior to jailbreak. You can then always go back to your original values.