DESCRIPTION:
Freezing is expleined in the manual here - http://www.image-line.com/support/FLHelp/html/recording_audio.htm#Jump_Freezing

Freezing is the process of converting real time CPU intensive synthesis and effects to much less CPU intensive Audio Clips.

Additionally, freezing can be used to apply sample-slicing and other 'resampling' effects to the audio or simply to tidy up a messy Playlist.

Enjoy