Capacitor C1 is charged through timing resistor R1 when the clock output is high. When C1 reaches the upper threshold voltage, the output signal decreases, and then C1 discharges through R1 until its voltage reaches the lower threshold point. When this happens, the output increases again and the cycle repeats itself. Using the parts values shown results in a frequency of 1 Hz.
The output frequency can be adjusted by trimming the value of R1