An (Signature HaSH) is a unique digital signature that Apple uses to verify the firmware version you are trying to install on your device. Every time you restore or update your iPhone via iTunes or Finder, the software sends a request to Apple’s servers. Apple then "signs" this request with a blob specific to your device's unique ID (ECID) and the specific iOS version.
Apple checks if that version is still "open." If it is, they send back an SHSH blob. shsh blobs
Many jailbreaks are only compatible with specific, often older, versions of iOS. Saving blobs allows you to "hop" to those versions later, even after Apple has closed the signing window. How the Process Works An (Signature HaSH) is a unique digital signature