This led flasher circuit works as a bidirectional shift register for a line of six led. Since the outputs of the CD4017 are connected to PNP transistors the led line takes the inverted signals from the outputs. In other words each led is ON when the related outputs of the CD4017 are OFF.
As we know the CD4017 features ten outputs, so it can drive a line of six led in bidirectional sequence (for more led a second counter is required, as I did in this circuit). The outputs from 0 to 5 drive the led line from L1 to L6; the outputs from 6 to 9 drive the led line from L5 to L2, then the circuit starts the next cycle. Note that the circuit you see in the video works by a 22uF capacitor and not 15uF as stated in the schematic.