The name says it all: the brake lever lock is placed on the front brake lever. Simple enough. By keeping the front brake lever pulled - because that's what it does -, your motorbike or scooter is restrained from being wheeled away.
These locks have a number of advantages. They are very compact and can often be carried under the saddle. Potential thieves will quickly notice that they won't get far by riding it and moving a heavy motorcycle with a locked front brake is not easy. Also, due to its very obvious location in the throttle area, you won't try to drive off with the lock in place, something that can happen with an out-of-sight brake disc lock.
There are disadvantages too. An impulse thief can already be sitting on your bike before seeing the brake lever lock. Also, having the front brake engaged for a prolonged amount of time may not be good. Motorcycles with ABS systems in particular (which is pretty much all of them these days) don't like that. Most brake lever locks have some adjustments or settings to prevent the brake from staying on too long.