It means you can do more at once at core speed. Each core will handle a thread, so you can run two things at once at 2.4ghz or whatever. If you have an app that uses both cores, then you can run twice the code and calculations at 2.4ghz. You might find some instances where it would SEEM like multiplying the speed, but in most cases you won't see it. The more cores, the less bottlenecks you have that stop your apps from accessing the fullness of a particular core. Just makes it more efficient.
I think that's how it works anyway