The six pins
Six pins, and two of them do nothing at all. GND is shared by all three lights, and RED, YELLOW and GREEN each drive one LED and nothing else. There is no supply pin: the pin your sketch drives is also that LED's power.
What is on the board
Seven parts. Three small surface-mount LEDs stacked red, yellow and green beside a traffic light drawn in the silkscreen, three resistors below them, and a six-pin header along the bottom edge. The header is right-angled, so the pins point out past the edge of the board and it lies flat in a breadboard or on a cable.
The six names
GND is the return path, on the square pad, and the only thing the three lights share. Each light's current leaves your board on its own pin and comes back here.
NC means not connected, and on this board it means it literally. The net list, which is the list of every electrical connection on the board, does not mention either NC pin. They are there so the header keeps the same first positions as every other TinkerBlock block.
RED, YELLOW and GREEN each go straight to one LED's anode, its positive leg. Set a pin HIGH and its light comes on; LOW and it goes out. The back of the board says this in as many words.
Each pin is its own light's power
There is no VCC pin. The pin your sketch drives supplies the current for its LED, the same way the XL LED's SIGNAL pin does. The XL LED's four pins goes through what that means for one light; here it is true three times over.
Two things follow. Each light's brightness depends on your board's logic voltage. And the three lights are three separate circuits, so your sketch can light any combination of them, including one no real traffic light shows.
The back of the board
Turned over, the order reverses. The square pad is on the right, and it is still GND. The back prints the part number, TK03, which is the number on the box and the address of this block's page, and two lines that are the whole manual in brief: each GPIO controls one light, HIGH turns it on and LOW turns it off.
When it does not work
Hold the board with the LEDs facing you and the header pins pointing down. Read GND, NC, NC, RED, YELLOW, GREEN from left to right. GND is also the square pad, which is the one mark that survives when the labels are hidden in a breadboard.
No. They are not connected to anything on the board, on either side. They are there so the first positions of the header match every other TinkerBlock block's. Wiring them does no harm and changes nothing.
There isn't one. Each LED is powered by its own signal pin: when your sketch sets RED HIGH, that pin supplies the red LED's current. That is also why the brightness depends on your board's logic voltage, 5 V on an Uno and 3.3 V on an ESP32 or a Pico.
Turned over, the order reverses: GREEN is on the left and GND on the right, and GND is still the square pad. The back prints what the pins do instead of their names. Count from the square pad and you cannot get it backwards.
Three loops that share one wire, and why nothing on the board stops all three lighting.
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