A reed switch is an electrical switch which is operated when a magnetic field is brought near to it. 

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Digital Output (DO)

GND: In electronics, we define a point in a circuit to be a kind of zero volts or 0V reference point, on which to base all other voltage measurements. This point is called  ground or GND.

3.3V  :  'VCC' stands for Voltage Common Collector. We'll connect the VCC pin to 3.3V on the micro:bit

Voltage is the difference in potential between two points. As it is difficult to talk about voltage without a reference point, we need another point to compare it to.