It just means that you have two cores that can work on different things at 3ghz, instead of using half of a one core processor to download a huge file while you try to play a game with the remaining power of that one core. It's about multi tasking.
If you have two 300 hp vehicles, it means you can go to the grocery store quickly while someone else uses the other car to go to the mall quickly, and you get two things done quickly at the same time. but you don't have a 600hp super car that can do it all at once.