Imagine a trigger guard on a gun, but for a light switch. This guard prevents the switch from being accidentally flipped, and also acts to discourage flipping the switch in general. In my case, I have a switch in my apartment which turns off the outlet that my cable modem and router are plugged into. This guard allows me to flip the switch when I need to (to reset the router/modem when they're misbehaving), while making it much less likely that someone will inadvertently kill my internet connection when trying to find the switch for the lights.