freezing a track renders out an audio version of your track to temporarily replace the devices that generate or process sound in real time. you can't make any changes to the recording while it's frozen besides the basic mixer controls, but your collaborator will be able to hear the track in the mix without having the vst you used in its chain. when you want to make adjustments, you can either "unfreeze" (which returns the track to normal) or "flatten" (which commits to the rendered audio).

if you're dealing with a plugin midi instrument, you might find it useful to duplicate the midi track, then freeze one and mute the other. this will ensure the instrument can be heard as intended but edits can be made to the notes.