The capacitor charges until the PNP transistor (here shown as a 2N3906, but you could also use a BC327) receives base current through the Zener and turns on. Then the NPN transistor (here shown as a 2N3904, but you could also use a BC337) turns on and the capacitor is discharged through the motor. As the NPN turns on the 2.2K resistor starts to supply base current to the PNP and the circuit snaps on. When the capacitor voltage drops below about 1V, the the PNP turns off, the NPN turns off and disconnects the motor from the capacitor which starts to charge up again.
The voltage across the capacitor rises slowly as it is charging from the output of a solar cell. This voltage also appears across the Zener in series with the PNP base emitter junction. The 2.2 KOhm resistor is connected in series with the motor to t