The 555 timer IC is connected for Astable Operation, the clock pulses are fed to the 4017 IC via the 10K resistor. The 4017 is a 10 stage counter, therefore the sequence of the traffic lights is spread over 10 clock pulses, 4on red, 1 on red & amber, 4 on green and 1 on amber.
We need red on for 5 pulses, so we connect the red LED to pin 12 which is on for the first 5 stages of the counter. The green and yellow LEDs are connected to the nescessary counter outputs, as both LEDs need to be on for more than one count, we use diodes to avoid a short circuit situation between outputs.
The capacitor and resistor on pin 15 of the 4017 are used to reset the counter to zero (red LED on) at initial power up.