An energy barrier was a barrier made of energy to keep individuals or objects in or out of an area.
Physical energy barriers
Physical energy barriers blocked solid matter and could be used defensively as force fields. A physical energy barrier was used by the Silurians to trap the occupants of Cwmtaff within itself. It was invisible to the naked eye, but could be seen if it was hit, or when excited by the Doctor's sonic screwdriver. The barrier was red in colour and did not allow people to pass through it. It did, however, allow rain to pass through. It could also block out light, making it seem as though it was night. (TV: The Hungry Earth)
Holographic energy barriers
Holographic energy barriers used energy in the visible spectrum to alter what was seen. The Terileptils used holographic energy barriers to create the illusion of additional walls in a house. The Fifth Doctor was able to dismantle one using his sonic screwdriver, allowing him to his companions to simply be stepped through it. (TV: The Visitation)