There are generally said to be three basic kinds of encounters between police and civilians. These encounters are differentiated from one another on the basis of how much coercion is involved.
On one end of the spectrum is a consensual encounter between police and a civilian. This is an encounter in which no coercion is involved. The police officer is not in any way detaining the civilian, who is free to go at any time. These encounters are often simply random. They might result when a police officer simply speaks to a civilian while the officer is walking along on patrol. They might result when an officer is canvassing near a crime scene and simply asks the civilian if he or she has seen anything. The major characteristic of this sort of encounter is that the civilian is not being compelled to stay with the officer in any way.
On the other end of the spectrum is an arrest. In such an encounter, the officer has to have probable cause to make an arrest. If the officer has probable cause, they can forcibly detain the suspect and require the suspect to come to the police station. In such an encounter, the civilian’s liberty is removed on a relatively long term basis.
In between these two extremes is something called “investigative detention.” In this sort of encounter, the officer has some reason to suspect that the civilian has done something wrong, but does not have probable cause to arrest. The officer may coerce the civilian and force them to stop and talk. They may do things like frisking the civilian. However, unless they develop probable cause, they cannot hold the civilian indefinitely.