Police officers are authorized to make contact with citizens if they have reasonable suspicion to believe that a citizen is involved in some type of criminal activity. Officers then need "probable cause" to detain a citizen for further questioning (or before arresting someone). 

If stopped, some one has the right to politely ask the officer if he/she is free to go. If the officer says yes, the person is free to leave and is not legally required to answer any additional questions.