The radiator cap is the gatekeeper of the cooling system. The cooling system uses pressurized coolant (also known as antifreeze) to take accumulated heat away from the engine. Each radiator cap is rated for a certain amount of pressure it can hold (e.g. 0.9 bar, 13 PSI, etc).

If you drive without the coolant cap for any length of time when the engine gets up to temp it will boil over out of the reservoir. You will lose at least half of the coolant in the system if you drive like that for a few days. Big chance of gasket failure.