It’s the symbol for a “variable resistor,” which (for low power requirements (*)) is realized as a potentiometer with one lead connected to the wiper and the other to one of the ends. It’s common to connect the wiper to the other end while you’re at it:
but that’s mostly to avoid dangling leads (and if you use PCB-mounted potentiometers, you want to solder all pins for physical stability).
(in this circuit, the resistance controls how quickly the 2.2u capacitor discharges)
*) EDIT: for more on potentiometer ratings, rheostats, etc, see the subthread that starts here.