This can mean many things, but usually it’s that a plain old traffic stop has turned into a circus.
Officer A makes a stop for, say, expired plates. Further investigation reveals that the plates are assigned to another vehicle.
Driver says “I took those plates off my other car so I could drive this one,” which is a common but woefully misinformed statement. Driver says he has no identification, and refuses to get out of the car when asked (and, yes, the officer can lawfully order you out of your car). Officer A calls for backup.
Officer B arrives for backup, and engages the passenger in conversation outside the vehicle. Passenger is marginally more cooperative, but provides an account that does not match the driver’s. Passenger also has an odor of marijuana on him.
Driver refuses to permit a search, and he still refuses to exit the vehicle. Officer A asks for a K-9 to sniff the perimeter of the car.
Officer C arrives with his dog. Before the dog can be deployed, the driver has to be out of the car. Driver is convinced that three cops really can extract him from his car whether he wants to get out or not, and getting out under his own power will produce less wear and tear. Driver gets out. Officers A & B watch him and Passenger while Officer C’s dog has a go. After alerting on the car, Officer C has the dog get inside the car, and the dog alerts on the center console. Officer C looks inside and finds a glorious assortment of illegal drugs and paraphernalia.
Driver and Passenger are handcuffed. Driver still refuses to identify himself. Officer A calls for a unit equipped with a mobile fingerprint scanner. Sergeant D arrives with this. The scanner reveals Driver’s true identity, and also that he has a revoked driver’s license and three outstanding arrest warrants, one of them a felony. The felony is related to a case that Officer E, who happens to be on duty, is supervising. He responds to the scene.
Poof, five patrol cars, all out of one traffic stop. In this scenario, Officer E might take Driver into custody, leaving Officer A to tow the car and collect the evidence. Passenger, who elected not to be a dick, is released at the scene and transported to a truck stop by Officer B, where he can call someone for a ride. Everyone else goes back into service.