If your computer is slow or audio is starting to gap or stutter because of a lot of effects or time stretching, you can freeze a track to reduce the amount of processing the track needs. After freezing a track, the software uses a temporary WAV file that includes all effects and audio.
You cannot edit the effects or sounds on a track until you Unfreeze the track. ( Dragging a new sound on to a track will unfreeze a track ).
To freeze, right click on a track you'd like to freeze and choose "Freeze Track".
Alternatively, choose "Track" menu > "Freeze Track".