An "SHSH blob" is some bit of data Apple's server provides each time you perform a firmware upgrade or restore. Security measures require it. Each SHSH blob is unique to your combination of device+firmware version. When Apple releases a new firmware (which usually blocks known jailbreaks), they also stop providing SHSH blobs for the previous firmware, preventing you from performing a restore to the older version. If you back up SHSH blobs for the current firmware, it's possible to downgrade to this version at any future date. Even if there's never another jailbreak for any future firmware, you can always go back to this one and jailbreak again...but only if you saved the SHSH blob.