This case is about searches and seizures that are conducted without a warrant. Specifically, it was about whether the police would need to have a warrant in order to unwrap two packages wrapped in opaque green plastic and placed in the trunk of a car. In this case, the Supreme Court held that the police had no right to open and search the packages without a warrant.
In this case, Robbins was stopped while driving and the police smelled marijuana when he opened the car door. They searched the car and found the two packages in a baggage compartment under the trunk (such as where a spare tire might be kept). They opened the packages and found marijuana.
In this case, the Court ruled that the police had acted illegally. It ruled that warrantless searches of closed packages could only occur if the packages clearly contained illegal material. There was nothing, the Court said, about the packages that made it obvious that there was something illegal inside them. Therefore, the police could not open them without first getting a warrant.