It enables the GLWF for Stereoscopic monitor like this from NVidia 3D Vision:

http://www.nvidia.com/object/3d-vision-displays.html

Usage:
	ofAppGLFWWindow * window = new ofAppGLFWWindow();
	window->setStereo(true);
	ofSetupOpenGL(window,1024,768,OF_WINDOW);

If you set stereo true without the correct monitor the application will not run correctly.