If you think about TTL or CMOS schmidt gates in simple RC oscillators
it is quite simple. On power up capacitor is empty ("0") so output of the inverter is high. Capacitor charges through resistor till input reaches level for logical "1". In that instant, output is inverted to logical "0" so
capacitor discharges through same resistor. Once the capacitor is low enough, input is considerd "0" what forces output again to be "1" and whole cycle repeates.
If you add potentiometer in series with resistor, you can change frequency. If you replace resistor for two resistors in parallel and add diodes in series with resistors (oposite polarity for diodes), you can change independantly time to charge and discharge capacitor. This way
you can easily change duty cycle.