This case is about searches and seizures of private property taken from a car that has legally been stopped by the police. It has to do with whether the police can search such property without a warrant. In this case, the Supreme Court ruled that the police need to have a warrant to search private property taken from a car.
In general, the police need to have warrants to search private property. However, one exception to this is when the private property is in a car and may be driven away and destroyed. In that case, the police have the right to search and seize the property to prevent the destruction of evidence.
In this case, Lonnie Sanders was arrested while riding in a taxi. He had a suitcase full of marijuana in the trunk of the taxi. When he was arrested (based on a tip and on his past record), the police searched the suitcase and found the drugs. The Supreme Court ruled this search was unconstitutional. It ruled that the police had control of the suitcase and that Sanders, therefore, would have no chance to destroy the evidence. Because of this, there was no reason to have to search the suitcase prior to getting a warrant.