The two inverters in series are a Schmitt-trigger with positive feedback. The capacitor is charged and discharged by R1 and when its voltage reaches Vcc/2 then V2 switches polarity. The capacitor charges or discharges the peak amount of Vcc/2 so its peak voltage is half Vcc higher than Vcc of half Vcc lower than ground.
R2 allows the voltage of V1 to go higher than Vcc or lower than ground without being clamped by the protection diodes at the input of the first inverter.