This is an unstable circuit (self-excited multivibrator circuit). It alternately charges and discharges two groups of resistors and capacitors C 1, R2 and C2, and R 1, so that two NAND gates (actually used as NAND gates here) are alternately in the state of 0 and 1, forming a square wave.

The larger the capacitance, the higher the resistance, the longer the charging and discharging period and the lower the square wave frequency.