Resolution; literally the ability to resolve musical details. It is all about overtone information. Some of this is harmonics of the fundamental note, some aren’t, and some are the effect of noise occurring in between. Your mind has the power to sort out and recognize multiple sounds at the same time. You connect the dots; a sequence of tonal events becomes music and touches your soul.
The essence of Resolution is about not letting the overtones mess with each other, or be messed with by noise and other internally generated distortions or information in the record playback process. All this changes the information in the original signal, stuff that confuses even the best minds making it hard to ‘hear’ the music offering.
An example of the complexity of a single piano note is shown above. The pitch of this note is 26 Hz; it is the A0 note, and what is shown is the spectrum of the note from 10 Hz up to 1,000 Hz. All this information is required to be exactly reproduced for you to clearly identify the specific note and instrument. The assorted overtones, while equally spaced in frequency, will vary in amplitude and phase relative to each other. This signature is unique to the instrument and even the player. Changes in overtones will let the experienced listener tell the difference between a piano made by Bosendorfer and Steinway.
Your ear and mind are absolutely incredible in this process and while your high frequency abilities may diminish with age, as you see from the above chart. Resolution is not necessarily about high frequencies above 10 kHz. It is about what happens in the mid-range to the interrelationships amongst frequencies, and faithfulness, and integrity of those signals.