Welcome to ETO, jithinsmmb!
The first 555 timer in the circuit simply sends the clock pulse to the CD4017 decade counter. Every time the push-button is pressed, it triggers a pulse from the 555 into the clock of the 4017. When pressed once, the first output of the 4017 is switched on. The variable resistor determines the output frequency of the second 555 timer. The first variable resistor is presumably set to a high setting, which slows down the output pulse of the second 555, which slows down the wiper motor.
When the push-button is pressed again, the first 555 sends another pulse to the 4017 and switches on the second output. The variable resistor here is probably set for a slightly lower value, which increases the frequency of the output of the second 555, speeding up the motor slightly. This entire sequence repeats for every depression of the switch.
I hope this helps!
Der Strom