Training Stress Balance/Stress Balance (see also Form) represents the balance of training stress.
It is the result of subtracting yesterday’s Acute Training Load/Shor Term Stress (“fatigue”) from yesterday’s Chronic Training Load/Long Term Stress (“fitness”).
In general terms, if it's negative, you're fatigued, and if it's positive you're fresh.
It is claimed to relate to your form or freshness.