The top left button freezes and unfreezes agents put into the freezer. The top right (screen lock) button allows you to toggle the freezer between two display modes: sitting on the ground or locked into a specific part of the screen.
This is a BIG agent, you might want to put it in a corner of the screen you don't look at much.
If the Deep Freezer is removed, any agents that were frozen in it are returned to the world and unfrozen. All agents in the world will be unfrozen as well.