In his tweet Elon Musk said: "Rocket is extra toasty and hit the deck hard (used almost all of the emergency crush core)"
What is he talking about?
The landing legs have a honey-comb filler that can compress to absorb significant energy of a hot landing.
The legs are designed to bounce and take some impact on landing. For further absorption of shock they include non-reusable crush cores, that deform and absorb the energy but cannot bounce back.
They are made replaceable and it is preferable to use them up than to have the stage hit the deck.