An unfinalized track is a track that hasn't completed uploading properly. You can finalize an unfinalized track by clicking on the red Finalize button.
If you click to finalizing your own track, it will retry the upload from your local backup. If you click to finalize a guest's track, it will take that tracks audio that was uploaded successfully to dropbox during your session and place it in your dropbox folder.
Sometimes not all of the audio for a guest makes it to dropbox properly. This can happen if they have a spotty internet connection, are recording from an office space with a firewall that blocks dropbox traffic, or if they leave the recording early before the uploads complete. This means that when you finalize a remote guest's track, it may be incomplete.
If you have your guest finalize the track from their end, it will retry the upload from their local backup and work around the problem of the incomplete file on dropbox.
More details on recovering tracks that have failed to upload properly here: One of the tracks didn't finish uploading, how can I recover it?