Probable cause means a reasonable suspicion, based on the officer's experiences, that a crime is being, or has been, committed.

Whether a police officer can search a passenger in a car, depends upon whether the officer has probable cause to believe that evidence of a crime will be found on that particular person.

If the officer, for example, pulls over a car because the license plate tags are expired, they would likely not have probable cause to search the passengers in the car.

If, on the other hand, the officer pulls over a car that they have been pursuing (as in a car chase), they will have most likely have probable cause to search everyone in the car for things like weapons or evidence of some other crime.