If you find a member particularly interesting, visit their profile and click/tap the Add to Favorites bookmark icon. 

You can find your favorite profiles by going to "Matches" and then by applying the filter Favorites. 

To remove a match from your Favorites, simply visit the profile and click/tap the Remove from Favorites bookmark icon.

On the app, add or remove someone from your favorites by visiting the profile and tapping the star in the top right-hand corner of the profile.