A deflection barrier was a form of force field technology. When an object made contact with the barrier, it would be deflected from it at the same kinetic force. The Fourth Doctor described walking into a deflector barrier as "walking into a brick wall". (TV: Pyramids of Mars [+]Loading...["Pyramids of Mars (TV story)"])
Sutekh set up a deflection barrier around the Scarman priory to prevent interference to the construction of an Osiran war missile. The reanimated corpse of Marcus Scarman and the Servitors set up four generator loops in each compass point in the area. Ernie Clements, who'd been poaching near the priory whilst it was set up, was trapped in the area because of the deflection barrier. The Fourth Doctor eventually disabled the eastern point of the deflection barrier to access Clements' hut. (TV: Pyramids of Mars [+]Loading...["Pyramids of Mars (TV story)"])