Quantum locking is a process by which a superconductor is "locked" in place within a magnetic field, as a result of the Meissner effect. This property allows for a superconducting material suspended in a magnetic field to become fixed so that it will not re-orient itself without outside assistance. For example, if you tilt the superconductor at a 45 degree angle while it hovers in the air, it will remain at a 45 degree angle.
For more information on quantum locking, check out our article What Is Quantum Levitation (and How Does It Work)?, which also contains links to videos demonstrating the behavior.
The phrase "quantum locking" appears to have been coined by Guy Deutscher, a superconductor researcher out of Tel Aviv University.