After post race inspections from the FIA, the stewards have found both cars to have failed inspection of their floor planks.
The skid blocks (planks) located across the bottom of all the cars are designed to protect the chassis from the high loads of downforce experienced by the cars when they bottom out and touch the ground. This is where we see the sparks fly out from behind the cars when the board is in contact with the ground. While this feature is there to protect the aerodynamic parts of the car, the FIA mandate the amount this board can be worn down during a race, thus how low the ride height of the car can be with the loads of downforce it experiences.
Article 3.5.9 of the technical regulations lays out the highly detailed design and layout of the planks, as well as the amounts of excessive wear or movement from the board at the end of the race.
You can see the skid block (plank) across the bottom of the Mercedes here when it was lifted off the track in Monaco.
