Jack barrel contact diagram?
The jack is being used as a power supply switch. If the symbol used is the normal one for a stereo jack, the plug must have its tip and sleeve shorted together to make battery negative connect to ground when it is plugged in. The signal would be fed into the circuit via the ring contact. 

However I suspect that the circuit is drawn slightly wrong, and the battery should be connected to the ring contact. That way a mono plug could be used, as its sleeve should be long enough to touch the ring contact.