Can you elaborate on the word reusable? Aren't normal landing legs also re-useable? Not that I questioning your skill or anything(it's too good).
He means self built landing legs rather than just the off the shelf ones. They overcome the usual limitation of the standard one where you can make them as big as you like to protect the larger engines and they 'bounce' a little, absorbing landing better.
The problem was, you could only use them once as you couldn't fold them away. Now you can, so for the first time you can do a proper Space X flight, landing, fold the legs back up, refuel, payload, take off again.