Shadow activities let you record a new activity while hearing how it compares to one that you, or someone else, has recorded. The experience is just like you were joining the other person in a live activity, with a commentary updating you on how you're both doing.
To start a recorded activity, go to the home screen and tap the blue Social button with a pair of runners on it.
On the next screen, select My activities to find one of your own activities to use, or Friend's activities to look for an activity from a friend.
Pro tip: Any time someone shares an activity - by email, to tribe chat, or directly with you - you can tp on the shared activity and use that start a shadow recording.
We've chosen Friends' activities. So now just select anyone of your friends, and choose one of their recorded activities to race against - easy!