A more thorough explanation:

A stop-and-frisk is a brief police stop of a suspect. Before stopping the suspect, the police must have a reasonable suspicion that a crime has been, is being, or is about to be committed by the suspect. If the police reasonably suspect that the suspect is armed and dangerous, the police may frisk the suspect, meaning that the police will give a quick pat-down of the suspect's outer clothing. This frisk is also called a Terry Stop, derived from the Supreme Court case Terry v. Ohio, 392 U.S. 1 (1968).

The Fourth Amendment requires that a stop-and-frisk must comply with the Fourth Amendment, meaning that the stop-and-frisk cannot be unreasonable. According to the Terry court, a reasonable stop-and-frisk is one "in which a reasonably prudent officer is warranted in the circumstances of a given case in believing that his safety or that of others is endangered, he may make a reasonable search for weapons of the person believed by him to be armed and dangerous."

For example, if a police officer sees a person walking down the street with a gun in their hand, the officer has reasonable suspicion to stop and frisk the person to ensure that they are not a danger to themselves or others.

However, if a police officer stops and frisks a person without any reasonable suspicion, the stop-and-frisk is considered unreasonable and a violation of the Fourth Amendment. Any evidence found during an unreasonable search is subject to the exclusionary rule and will be excluded from being introduced at trial.

For example, if a police officer stops and frisks a person without any reasonable suspicion and finds drugs in their pocket, the drugs cannot be used as evidence in court because the stop-and-frisk was unreasonable.

It is important to note that a stop-and-frisk falls under criminal law, as opposed to civil law.