Parc fermé is the condition a Formula 1 car enters at the start of qualifying, after which teams may not change its setup before the race. Only a short list of permitted jobs is allowed. Breaking it means starting from the pit lane, so a fast qualifying setup has to survive the whole grand prix.
The name comes from the enclosed area cars are held in after a session, but the meaning that matters is the rule rather than the place. From the moment qualifying begins, the car is effectively frozen: wings, suspension settings and ride height are locked, and mechanics may only carry out routine work such as changing tyres and topping up fluids.
Teams cannot, therefore, set the car up for one lap and rebuild it for the race. A car trimmed for a fast qualifying lap has to survive a full grand prix on the same settings, so Saturday's grid position is bought with Sunday's pace.
Breaking parc fermé is sometimes the right call. A team that has misjudged the weather, or wants to change the car after a poor qualifying, can take the pit lane start deliberately and get a free hand on setup in exchange for losing track position.