Fitness in cycling is defined as an accumulation of training
Fitness is an exponentially weighted average of the last 42 days of training and reflects the training you have done over the last 3 months.
The workouts you did 15 days ago will impact your fitness more than the workouts you did 6 weeks ago. You may notice that as Fitness goes up so does Fatigue, only at a greater rate.
The Performance Management Chart tells the story of your fitness, consistent training is marked by a steady rise in Fatigue and Fitness whereas a sharp drop might indicate time off due to sickness or injury.