What you have shown is a voltage regulator circuit using a very common component. It does not take power line voltage as an input. As you can see from your drawing, it will accept from 8 volts to 18 volts DC as an input and provide 5 volts DC as a regulated output. That voltage may come from batteries or a transformer driven rectifier circuit.
The ground symbol represents a point that is common to both the input and output of this circuit. Other components in this circuit provide a means of turning it on or off, an indicator that it is on, and filtering to make the DC signal as steady as possible.
On edit.... Thanks Audioguru... I totally missed the solid. When you see a diagram like that the mind sometimes puts things in place that aren't really there......