There is a current (IB) flowing into the base of the transistor which switches the transistor on.now current can flow from the collector to the emitter
The transistor in this configuration is being used as an amplifier (small signal in, big signal out).
Your example is known as an inverting amplifier, which means that at the positive peak of the input signal you "should" see a peak negative signal (measured below the 3K3 resistor).
To provide a decent amplification of an AC signal (which alternates around 0 volts) the circuit provides a bias via the 390k resistor to lift the entire signal above 0 volts probably to 2.5-3 volts (you need to do this or you will only see the positive peaks of the signal).
A useful feature of this circuit is that the placing of the 390k resistor provides negative feedback which attempts to stabilise any variations in supply (and in a small way gain) variations.
When I first stumbled on this circuit there was loads of great help on this site, and plenty of examples of "better" amplifiers - but this one provides a useful starting point.
The "why" is that it works because it obeys the laws of physics (In the case of the transistor that includes the laws of semiconductor quantum physics).
Any explanation beyond that goes to the metaphysical or philosophical.
The "why" is that it works because it obeys the laws of physics (In the case of the transistor that includes the laws of semiconductor quantum physics).
Any explanation beyond that goes to the metaphysical or philosophical.