A transmat lock was device designed to prevent the use of transmat technology within an area. Multiple devices could be used to form a boundary in which transmat beams could be reversed and prevented.
The Thirteenth Doctor used four tansmat locks to prevent the Thijarians from transmatting into Umbreen farm after she confiscated them from the Thijarians. They were later able to break through the boundary created by the locks to transmat the Doctor away. (TV: Demons of the Punjab)