It means that a mix, has a well balanced harmonic and frequency spectrum within the 3 major densities (Low, mid-range & high end). Their frequency transients are not overshooting the normal limits, allowing for higher overall output levels without compromising dynamic range.

Our mastering process checks the alignment on 30 frequency bands, which is a high standard for most mastering facilities, and although good  frequency alignment and harmonic balance does not guarantee great  sound, it is necessary for accomplishing the best mastering results.