A security escutcheon is a steel plate fitted over the lock cylinder on the outside face of the door to stop it being pulled out, snapped or drilled. It is held by bolts that go right through the door leaf and are fixed on the inside, so it cannot be removed from outside.
As well as protecting it, it covers the cylinder if it sticks out, which is one of the most common weak points. There are fixed escutcheons, which leave only the keyway of the cylinder on show, and magnetic ones, which cover it completely.
Do not confuse it with a decorative rose: a rose only covers the hole and comes off by hand. For a home on the coast, choose a finish that stands up to salt air. You will find it under escutcheon installation.