High
The tank is empty and rest isn't refilling it. That's what separates this from ordinary tiredness: a full night's sleep or a free Saturday used to reset you, and now the effect wears off before the week has started. Work costs more than it returns, and the shortfall gets paid out of everything else — the evening, the people you live with, the things you meant to do. Bodies do this when demand outruns recovery for long enough, and they do it regardless of how much you care about the job. A high reading here is arithmetic about load and recovery time, not evidence about your stamina.
Moderate
Work takes something out of you and mostly gives it back. Hard stretches register — a heavy week leaves a mark, and you notice the days you arrive home with nothing left — but recovery still works: a weekend, a few lighter days, and the level comes back up. This is what a sustainable load feels like from the inside, not proof that nothing is wrong. It's also the band that moves fastest in either direction: another month at the current pace can push it up, and a genuinely protected week or two can pull it down. Worth watching rather than worrying about.
Low
The work costs what it costs, and you recover from it. Demanding days don't leave a residue that follows you into the next one, and time off does what time off is meant to do. That isn't stoicism or a sturdier constitution — it usually means load and recovery are roughly in balance right now, which is as much about the job as about you. Two things worth holding onto: this can change when conditions change, so a low reading isn't immunity; and people working the same hours beside you may not be getting the same return, which is worth remembering before offering advice.