It means that for your first 5GB, you get very fast 4G speeds. Once you hit that limit each month, though, you are throttled back to slower speeds. It's unlimited data, but it's not unlimited data at 4G speeds.
Different carriers throttle in different ways, though, so some might slow you down only during peak times. No matter what they advertise, I'm not aware of any carrier that truly offers unlimited wireless data at 4G speeds. As noted before, it would bring a carrier's network to its knees and is commercially unsustainable.