A Formula 1 car is locked into its configuration from the start of qualifying until the race finishes. That restriction is why teams commit to a setup before knowing what they will need.
The freeze applies to the car, not the track
Once a car leaves the pits for qualifying it enters a controlled condition in which most adjustments are prohibited until the race is over.
Permitted changes are narrow: front wing angle, tyre pressures within limits, and specified maintenance. Suspension geometry, gear ratios and aerodynamic configuration are fixed.
Weather does not lift the restriction in the way teams would like, so a setup chosen in dry conditions must be raced in whatever arrives on Sunday.
Qualifying and the race want different cars
A single lap rewards maximum downforce and a car working in a narrow window at peak grip.
A race rewards tyre preservation, stability over a full fuel load and less drag on the straights where overtaking happens.
Because one configuration must serve both, every setup is a weighting between grid position and race pace, and teams differ in which they prioritise.
Practice becomes the only real testing
With in-season testing heavily restricted, the practice sessions before qualifying are where a team gathers the data to make that choice.
Running is therefore split between low-fuel laps that indicate qualifying pace and long runs that indicate tyre behaviour over a stint.
A session lost to rain or a mechanical problem removes half the picture, and the resulting setup is closer to an estimate than a decision.
Breaking the freeze carries a defined penalty
Teams may make prohibited changes if they accept a pit lane start, which sacrifices grid position entirely.
This becomes worthwhile when qualifying went badly or when conditions changed so completely that the existing setup is unusable.
The trade is explicit and calculable, which is why the decision is usually made calmly rather than as an admission of failure.
Why the rule exists
Without it, teams would run a qualifying specification and then rebuild the car overnight into a race specification, which requires staff, parts and cost.
Freezing the car limits that expenditure and reduces the advantage available to the teams with the largest operations.
It also makes the grid meaningful, since the car that set the time is the car that starts the race, which is not true in every category.