Apple "signs" iOS versions for a very short window (usually 1-2 weeks after a new release). Once the signing window closes, you cannot downgrade.
In the world of iOS customization, (Signature Hash Blobs) are essentially the "digital keys" Apple uses to control which versions of iOS you can install on your device. What are SHSH Blobs? shsh blobs
Wise travelers know they must "save their blobs" while the window is still open. They use specialized tools to trick the server into giving them a signature even if they aren't ready to use it yet: Apple "signs" iOS versions for a very short