Also known as the Faraday's first & second laws of electromagnetic induction.
First law: whenever the magnetic flux linked with a circuit changes, an EMF is induced in it. Second law: the magnitude of that EMF equals the rate of change of flux linkage — the flux change per second times the number of turns.
Move the magnet and the meter kicks; hold it still and the needle drops to zero.
EMF = −N × dΦ/dt
EMF
induced voltage (volts)
N
number of turns in the coil
dΦ/dt
rate of change of magnetic flux (webers per second)
In plain English: A changing magnetic field through a coil creates a voltage. Change it faster, or add more turns, and you get more volts. A steady field induces nothing — it's the change that counts.
Where you'll meet it: The working principle of transformers, generators, guitar pickups, induction cooktops and wireless charging. Also why an inductor fights any change in its current.