Electronic Stability Control. The switch temporarily disables it, so you're actually turning it off; the default is on. You won't feel any difference except briefly, in very specific road conditions. LINK.
ASR and ESP are automatically active when you start the car. There is a switch provided so you can shut them off if you need, but they will automatically be active again the next time you start the car. Obviously, you can also re-engage those systems with the same switch used to shut them down. When you hit the switch and the light comes on, the system is off. The light is on to remind you that the system has been deactivated because normally these systems are always on.
Much like ABS, you usually leave these systems alone and they sit there in stand-by mode waiting for something to go wrong at which time they will step in and take action to correct what they can. Unless you break traction or are about to break traction, these systems don't do anything. The option to shut these systems off is provided because you may run into a rare situation where the system is hindering instead of helping.