This is your "eyes" in the world of electronics; it lets you see invisible voltage and current.
Electronic circuits are built from a handful of standard parts, each with a unique job: Electronics For Dummies
Tiny switches or amplifiers that are the "brains" of modern computers. This is your "eyes" in the world of
One-way streets that allow electricity to flow in only one direction (LEDs are a famous type that light up!). Electronics For Dummies