He means illegal without a warrant.
Having just read the opinion, that's not actually what they literally said. The Court rules that using a GPS locater on a person's car was a "search." The Court expressly said:
The Government argues in the alternative that even if the attachment and use of the device was a search, it was reasonable—and thus lawful—under the Fourth Amendment because “officers had reasonable suspicion, and indeed probable cause, to believe that [Jones] was a leader in a large-scale cocaine distribution conspiracy.” Brief for United States 50–51. We have no occasion to consider this argument.
Usually a search requires a warrant, but there are exceptions, one notably being the "
automobile exception" to the warrant requirement.
If the Supreme Court meant to overturn the automobile exception (or deem it inapplicable in GPS cases, it's very strange that they didn't say so. Actually, it's strange to me that it didn't come up at all even if they didn't mean to make an exception to the exception.
It's a strange ruling. It probably means that a warrant is required, but that is reading between the lines and to some extent assuming the automobile exception no longer applies. If courts rule that this ends the automobile exception, police can't search vehicles without a warrant. My guess if that courts will rule this to be an exception to the automobile exception, but that every other search of a vehicle is exempt from the warrant requirement. That's a weird rule though, because the law is clear that on probable cause they can break the lock on your car door and the glove compartment, then rummage through your personal effects in the car, all without a warrant, but they cannot attach a small device to the outside of the car.
I'd also guess that a split between the circuits will eventually arise.