Follow-Along is useful only when playing back a completed transcript. Use this to play your transcripts back and have the transcript view "follow along" by highlighting the currently-spoken text.
When Follow-Along is enabled, the play head follows the currently-selected entry and vice-versa. In other words, if you press play, as the playhead sweeps across the track and encounters entries (the time code is greater than or equal to the next entry's time code), the entry is highlighted (and scrolled to) in the editor view.
Note: When you enable Follow-Along the editor is disabled (you cannot edit or modify transcript entries).