the freeze terminology comes from a few other apps and it generally refers to processing the track with all its effects and plugins and saving a background pre-built track that plays in place of the actual track data. This allows otherwise cpu overloaded situations to playback since the frozen track is no longer going thru the actual math to process the data in realtime... simply playing back an already processed mix file in the background. Slightly different from a bounce situation where you are generally blending and mixing a group of tracks together as a stem and introducting that into the mix engine instead of the individual separate tracks.