The 555 is running in the astable mode at about 145 Khz. It is clocking the 7474 at this frequency. The 7474 is set up as a bistable D flip-flop. It keeps changing state for each clock pulse. When PB1 button( the push button connected to pin 4 of the 555) is pressed it stops the 555 from generating a clock pulse and which output of the 7474 is low lights up the LED connected to it's pin.