Changes the "current folder" to the folder whose pathname (full or partial) is given by the string. Returns #t. With no argument, opens a dialog box which you may use to select a folder. Will not work when the string is more than 255 characters long, or when the string is not a valid pathname (full or partial) to a folder.
The "current folder" is where Pixie Scheme operations look for files when you do not provide a complete pathname.