A graphic representing the edge of the magnetic shield on Rura Penthe
A magnetic shield is an energy field which can prevent the use of a transporter.
Such a shield was in use at the Klingon penal colony Rura Penthe to prevent ships from beaming prisoners away from the prison. In 2293, Captain Kirk and Doctor McCoy were able to make the long and dangerous journey across the surface to escape the shield's influence, where they were subsequently beamed up by the USS Enterprise-A. (Star Trek VI: The Undiscovered Country)