Fatigue (Also known as freshness, Acute Training Load, or ATL). By taking an exponentially weighted average of that stress from the past 7 days we are able to calculate your Fatigue, or an estimate of your fatigue accounting for the workouts you have done in the past two weeks.
Here you can see on days with a workout with a high training stress score the Fatigue climbs sharply in response. You do a hard workout today and you will feel it in the coming days.