The capacitor is placed in parallel and when the switch is pressed it is charged. When the switch is released the capacitor discharges but as this happens it holds the relay closed for 3/4 seconds. This allows the computer enough time to detect the closed relay and then it turns on a motor.

The 555 circuit shown above is more sophisticated than the circuit above and is composed of several components included the integrated circuit (NE555). When switched on the buzzer sounds for a certain amount of time.
Some of the components are resistors and capacitors. It is often the combination of resistors and capacitors that control the time delay - in this case the length the buzzer sounds for.
If the capacitor C1 is changed for a higher value capacitor then the buzzer sounds for a longer period of time. The variable resistor VR1 can also determine the length of time.
 

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