The circuit diagram shows you the structure of the clock. It is composed of two NOT gates, a capacitor and a resistor. How it exactly works is not imortant at this point. Importantly, it provides a periodic clock signal with a frequency of about 1 Hertz.
Now the circuit adds every second a 1 to the stored value and does this tirelessly from 0 to 15 over and over again.
For now it is enough on counter. Next, you have a look at shift registers.