Playback is an intelligent tracking feature in that it shows you the entirety of a particular journey playing out on a map with stats like speed in order to show you exactly where you went and how fast, you are also able to view your journey from a satellite perspective, giving a detailed view of your trip.
Menu (Top Left) > Report > Playback
You can find how to access and use Playback here: https://www.youtube.com/watch?v=9TifFly4mu8