This last circuit is using a 555 timer chip to make noise using a speaker.
What is happening is that the configuration of components and connections on the 555 chip is causing pin 3 to oscillate rapidly between high and low. If you were to graph these oscillations, it would look like a square wave (a wave the alternates between two power levels). This wave then rapidly pulses the speaker, which displaces air at such a high frequency that we hear this as a steady tone of that frequency.