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 and protect planetoid colonies from orbital bombardment.
In 2256, during the Federation-Klingon war, the mining colony on Corvan II utilized such shields as a defensive measure to counteract surface bombardment executed by Klingon forces in a surprise attack on the colony. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")
A magnetic 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)
The Aldeans also used a magnetic shield to block transporter functions, as part of their reclusive tendencies. In 2364, upon revealing themselves to the crew of the USS Enterprise-D, the Aldeans kidnapped a number of children from the ship, in an attempt to reinvigorate their culture. In response, the Enterprise crew conducted an analysis which revealed momentary fluctuations in the shield. Isolating these fluctuations, an away team was able to beam down and disable the shield, successfully gaining the upper hand. (TNG: "When The Bough Breaks")