Search Incident to Arrest is an exception to the 4th amendment's protection against unreasonable searches and seizures. That means that if properly followed by law enforcement, officers can search without a warrant. Search Incident to Arrest means that once a valid, legal arrest has already been made, for any lawful purpose whatsoever, the officers may then search the arrested individual completely, without a warrant. This includes all pockets and property in reasonable proximity to the arrest, including a car and all locked areas of the car. This typically does not include a home, if the person is arrested near their house.
Search Incident to Arrest often yields drugs or weapons after an arrest, since those items are easily concealed in pockets or a jacket. The warrantless search is widely used and often lands the defendant in jail with several additional charges added to the original offense for which the arrest was effected.