Progression is the how your fitness has been improving or declining over time. Xert determines your fitness activity-by-activity. It uses the previous activity's fitness signature as the basis to determine your fitness for the current activity. If you haven't been training and doing any activities for a given period, Xert will consider this a period of detraining and will reduce your fitness automatically.
To keep Xert on track with your current fitness, be sure to include maximal efforts into your activities.  These express your fitness and Xert will pick these up and use them to adjust your fitness up or down accordingly.