This is the symbol for 'the ground'. The ground sometimes represents one pole in some circuits. With a little bit of fantasy you can recognize that there's simply something connected with the earth - or with the ground.

Well, this little buddy connects us to the floor, you think? Yes, this is true. And it is also the reason why you instantly die if you get connected to one pole of a high-voltage, high-current power source with your feet (or anything) touching the ground. And this is also the reason why it is possible for birds to sit on power lines and drop their things onto your car without getting roasted. They just keep away from the ground. (sensible little animals, those birds, aren't they ?)