The United States and Pennsylvania Constitutions protect citizens from unreasonable searches and seizures. A police officer may briefly detain a citizen, without probable cause, for investigatory purposes in certain limited circumstances. This type of seizure is known as a "Terry stop" after the United States Supreme Court case, Terry v. Ohio.
In order for a Terry stop to be reasonable under the constitution, the seizing officer must point to specific and articulable facts, which take with rational inferences lead a reasonable officer to believe that criminal activity may be afoot. A Terry stop is unconstitutional if the officer is acting merely on a hunch.
Once a citizen is seized for a Terry stop, the officer may then conduct a "Terry search," if he has reason to believe that the citizen is armed and may pose a threat to the officer's safety. If the officer can point to facts that lead to a reasonable belief, the officer may perform a patdown to search for weapons during the Terry stop.