Smart Set is a Premium Feature within the free swim experience. 

While swimming a free swim, Smart Set will show you the sets you have completed so far. Previously, the free swim experience would only show your completed distance if you chose that metric to be in your dashboard. Now, with Smart Set, you'll see your completed distance so far, as well as a count of how many repeated intervals you've done (e.g. 5 x 100m FR) so that you have more space for other metrics.

Example screen: 

In the middle pane, you will see your interval count and, underneath, the total distance so far for the workout.

You can turn off Smart Set in the Advanced Settings of the app under Goggles then Goggles Settings.