This means that when current is applied to the coil it throws a simple changeover switch, terminating the connection from the NC contact to ground and closing the NO contact. Use them to switch high voltage/high current devices.
The first paragraph from the relay tutorial:
Microcontrollers are a ton of fun. Once I got hooked, there was no turning back. Initially playing with sensors and LCDs, I quickly discovered the limits to what a microcontroller could control. A microcontroller's GPIO (general purpose input/output) pins cannot handle higher power requirements. An LED was easy enough, but large power items such as light bulbs, toaster ovens, and blenders required more sneaky circuitry. Something sneaky called a relay:
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