A locking solenoid is a conventional solenoid, a wire coil that is magnetized when paired with an electrical current and locks a door or device. The coil is small enough to fit into a lock, where it keeps the locking mechanism from moving unless an electromagnetic force is used to gain access.
When a door or device locks via a solenoid, it is using basic electromagnetic forces to control the lock. The solenoid fits in the locking mechanism and, when locked, will expand so the device cannot be unlocked by sheer force.
While the locking solenoid will keep the device locked, it is not technically on when in locking mode, because no power is being used. The solenoid only needs power when unlocking and, because most devices are consistently locked, very little energy is ever required by the solenoid.
For an example of a solenoid lock, take a look at the GTO FM144 Automatic Gate Lock.