A security escutcheon is there to protect the cylinder on the outside. It is a steel part screwed on from the inside of the door that covers the cylinder, leaving only the keyhole on show.
That way the cylinder cannot be gripped, snapped or pulled out, and it is much harder to drill. If the cylinder sticks out slightly from the door, the escutcheon covers it and removes that weak point.
What matters is that it is properly fixed: the screws have to go in from the inner face, so there is nothing to unscrew from outside.
It works best with a security cylinder, because the escutcheon protects against force, but not against every way of opening. It is explained in escutcheon installation.