In this case, Wilson was a passenger in a car which was stopped for speeding. The car took a while to actually stop, the passengers kept ducking out of view of the officer, and the driver (when he finally did stop) jumped out of the car, met the officer halfway, and was generally acting like a spaz. All of this made the officer suspicious, and he ordered the passengers out of the car. When Wilson got out, he dropped some crack. He was arrested, but the trial court held that it was unlawful for the officer to order Wilson out of the car because he was only a passenger. Maryland appealed.
The US Supreme Court expanded it's decision in Pennsylvania v. Mimms to include passengers, using pretty much the same reasoning. In other words, the court held that it is reasonable for officers to remove passengers from the car on traffic stop because the government's substantial interest in the officer's safety outweighs the minimal potential inconvenience to the passenger of waiting out the detention outside of the car instead of inside it.