Hi @Gerry15 and welcome to the Fitbit Community.
A walking pace of 21' 37" means that your average walking pace is 21 minutes and 37 seconds per mile.
So, if you maintained that average walking pace for a 1-mile walk, it would take you 21 minutes and 37 seconds to complete the walk.
Your average walking pace can be used to estimate how long it will take you to complete a specific distance, or it can be used to see if your average walking pace decreases over time (a potential indicator of improved fitness).
One limitation of walking pace, however, is if you do walking with hills / stairs / elevation / incline. The average person has a slower walking pace when walking a route with hills than walking a flat route, so it's not fair to compare your average walking pace for a flat walk with your average walking pace for a walk with stairs or hills.
Does that help?
