A loudspeaker converts electrical waves into mechanical energy by moving a diaphram, thus creating sound.

This circuit is for low power speakers.

A loudspeaker normally has a very low resitance - between 4 and 64 ohms, 8 ohms is most common.

The capacitor is recommended to prevent any DC current flowing causing over heating.

A 555 astable (oscillator) will drive most small speakers directly via a capacitor.

If this is connected to an Audio Amplifier and it already has a capacitor in series with its output, you can omit C1.