A: When Apple releases a new OS, they sign it. This means that the version can be officially updated/downgraded to from iTunes. When the new versions are released, there is a window of time where you can upgrade/downgrade to them. This is called the “signing window”.
A: A blob is the term for a small piece of data that controls whether an iOS device can update to a certain firmware version or not. It allows you to install other iOS versions without them being signed, just as if they were actually signed. You can save blobs on tsssaver.1conan.com.