The court just ruled that police can no longer search a car incident to arrest... assuming the car isn't within reach of the arrested person and there is no reason to suspect that the car contains evidence related to the arrest.
Since New York v. Belton (1981), police have assumed that they can search a car any time the driver is arrested. This relates to Chimel v. California (1969) saying a search incident to arrest is justified by officer safety or the interest in preserving evidence.
In Arizona v. Gant (2009), a man was arrested for a suspended license and in custody in a police car. Police, because they could, searched his car (officer safety?) and found drugs. This is what has been overturned.
The real-world implications of Grant will be small. In my experience, most searches of cars happen not incident to arrest but technically to "inventory" belongings when the car is towed. Grant does not address that.
But I'm always pleased whenever the courts extend fourth amendment freedoms of citizens. It doesn't happen too often.