This case was about the rules that govern searches and seizures of property from cars that have been stopped by the police. In this case, the Supreme Court affirmed at that the police have broad powers to search the contents of cars without first getting a warrant.
In this case, the police had watched bags of marijuana being brought into an apartment building. Later, a man named Charles Acevedo left the building carrying what appeared to be one of the bags. The police saw him put it on his front seat and drive away. They stopped him and searched the bag. They found marijuana and arrested him. He claimed the search was illegal.
At that time, there were confusing rules about searching closed containers in cars. If the police had probable cause to stop a car, they could search closed containers inside it without a warrant. However, if all they had was probable cause to search a closed container, they could not search it without a warrant even if it was in a car. This meant that it was not always clear when police could search containers in cars.
In this case, the Supreme Court ruled that the police may always search a container that has been seized from a car if they have probable cause to believe that it contains contraband or evidence. This did away with the confusing rules that had previously existed.