What it actually means.
A police officer witnessed three men pacing in front of a jewelry store and suspected that a robbery was being planned. He approached the men and identified himself, then performed frisks of defendants Chilton and Terry and discovered illegal concealed weapons. Defendants were convicted and appealed, claiming that the frisk violated their Fourth Amendment right against unlawful searches and seizures.
The Supreme Court upheld the conviction, finding that when a law enforcement officer has "reasonable grounds" for suspecting that a criminal suspect may be armed, he may pat down the outer layer of the suspect's clothing for weapons. The ruling held that the Fourth Amendment protection against unreasonable searches and seizures is not violated when a pat down is performed based on reasonable suspicion for the purpose of ensuring officer safety.