It means that the entire CPU i.e. all cores run at 2.5 GHz. That doesn't make it 10 GHz.
Unless you follow the ebay rules of advertising PCs
Also note that 2.5 does not equal 2.5 i.e. amd's 2.5 is not equal to intels 2.5, and ivy bridge 2.5 is not equal to sandy bridge 2.5