iPhone SHSH Blobs:
Every time you install iOS onto your iPhone it 'phones home to apple' for a special response, this submits your phones unique ECID and the firmware version you are about to install and looks to get a valid response back from apple (this is the SHSH blob). If the response is Invalid this generates an error in itunes and makes you unable to install the version of firmware you are attempting to. As newer versions of firmware appear apple stops signing old versions of iOS.
To continue to have the ability to return to old versions of iOS you require a way to save your SHSH blob. This is where TinyUmbrella comes in (http://thefirmwareumbrella.blogspot.com) download for your OS run, connect up your iphone and it will save your SHSH blob locally and also upload to cydia (sauriks shsh store).
This will guarantee you the ability to always install that version of firmware.