The file .TERRData is created in the current directory by the function save.image() when you quit TERR and specify yes when prompted to save an image of the session.

.TERRData contains binary representations of TERR objects. On startup, if TERR finds this file in the current directory, it calls load("./TERRData") to load its objects into the global environment and displays a message that the previously saved workspace is restored.