The four pins

What is on the board

A 12 mm tactile switch under a blue cap, two small resistors, a red LED, and a four-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 switch is a USAKRO 12 × 12 mm part with 0.3 mm of travel. Under it, the two resistors are the 10 kΩ pull-down, on the left, and the 1 kΩ for the LED, on the right. The LED is the small part between them and the header.

The four names

GND is on the square pad. The pull-down and the LED both return here.

VCC goes to one side of the switch and nowhere else. That is unusual enough to have its own article: whatever you connect to VCC is what your pin receives when the button is down.

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 it at all.

SIGNAL is the output. Released, it sits at 0 V and your board reads LOW. Pressed, the switch joins it to VCC and your board reads HIGH. The silkscreen says so on the front: ACTIVE HIGH.

Not a pull-up board

Many button boards are wired the other way round, with a resistor to the supply and the switch to ground, and on those a press reads as LOW. Sketches written for them test for LOW, and plenty of them are online. On this board that sketch reports a press all the time you are not pressing. The pull-down explains the difference in one figure.

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 TK04, the number on the box, and WORKS ONLY WHILE PRESSED: this is a momentary switch. Let go and SIGNAL goes back to LOW.