Specifically they ruled that bringing a dog onto a property to nose around in an attempt to find evidence to get a search warrant is a violation of 4th amendment rights.
Privacy, Property, and the Drug War
Jardines v. Florida involved a home search that uncovered marijuana plants, leading to a conviction. The police received an unverified tip that marijuana was being grown in Jardines's residence, but presumably did not believe that this tip was sufficient to establish the "probable case" required by the Fourth Amendment to obtain a search warrant. To obtain a warrant, the police took a drug-sniffing dog, who indicated that it had found the scent of illegal drugs. The findings of the dog's actions were used to obtain a search warrant to search Jardines's house, which led to the uncovering of the marijuana plants and his subsequent prosecution.
The question in this case is whether the dog sniffing around the outside of Jardines's house constituted a "search" within the meaning of the Fourth Amendment. To state the obvious, evidence obtained in a search cannot be used to retroactively establish the probable cause necessary to obtain a search warrant. The state of Florida argued that because the dog did not enter the house but merely searched the exterior that was visible from the street, the dog sniffing did not constitute a search. In a 5-4 opinion, the Supreme Court rejected this specious argument. Justices Scalia and Thomas—whose sporadic commitment to textualism does sometimes surface in Fourth Amendment cases—provided the swing votes, with the former writing for the majority.