What does it do? A pulse longer than 500ns on the input will turn on the LED, a shorter pulse will turn it off. Pulses received after the first pulse will be forwarded to the next device. If no pulse is received for 20µs, the first device will be ready to receive pulses again.
Sounds complicated? In the end this allows to use a train of pulses to program the state of a number of LEDs connected in series. The same principle is used to control the WS2812 addressable RGB LEDs. The difference is that this circuit is implemented entirely using discrete transistor logic.
See it in action here
Longer design description: https://cpldcpu.wordpress.com/2021/02/27/the-transistorpixel/