Degrees of holiness

When they say that the Temple Mount is holier than the Western Wall or the Mount Hermon Regional Council, what do they mean?
Is holiness something that exists that we simply don’t know how to measure with physical instruments, or is holiness simply a law in halakha about how one should behave in certain places, times, and objects, or is holiness related to the chance that God will hear prayers, just as in the Temple Solomon prayed that God would listen very carefully to everyone’s prayers there?