Your ability to ride a bike quickly (or for a long time) on any given day is governed by two factors: fitness and fatigue*.
(* Okay, there are more factors, but let’s keep things simple).
Your fitness dictates how attuned your body is to fulfilling the demands that you want to place upon it.
Your level of fatigue (or tiredness) dictates whether your body will perform at this level of fitness on the day of your event (or hard training session).
Under this simplistic model, fitness minus fatigue equals form (i.e. your ability to perform on the day).
The Strava Fitness and Freshness chart is a visual representation of these three factors, plotted over time. It could help you work out if you’re fit, knackered or pulsating with athletic vim(to).