What is "seal off"

Usage examples of "seal off".

Murdock went to the next target in line, the airfields that would launch the three-thousand-paratroop drop to seal off any opposition and capture the port facilities needed to land the invasion troops.

Once the rocks were in place, he had them pile dirt in front of them to seal off the damn.

Damnit, Iran could seal off the coast from them with five thousand troops if they really wanted to.

Then it would be fairly simple to seal off a compartment and hide the warheads there.