FitnessAlerts are Runkeeper's way of letting you know when you've reached or passed a certain milestone in your workouts. From letting you know if you've just completed your longest activity ever, to letting you know if you've just set the bar for your new fastest average pace, FitnessAlerts will help you keep track of your personal records. They will appear in your FitnessFeed after you perform a record setting activity.
A more complete view of your Personal Records can be found in the Reports tab. Just select which activity type you'd like to view from the left-hand menu, then scroll down and view your Personal Records!