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 using this guide.