The Supreme Court has ruled in the past that if you are arrested (not just a traffic stop, but an actual arrest), the police can search you without a warrant and use as evidence anything they find on you. Today's ruling says that that rule no longer applies to cell phones; the police can seize them if you're arrested, but can't search them until they get a warrant.
Kinda a funny distinction given the actual wording of the 4th:
Amendment IV
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.