In 1831, Michael Faraday found something that shouldn't have been obvious from anything known about electricity or magnetism up to that point: moving a magnet near a coil of wire — with no wires touching, no battery involved — caused a current to briefly flow through the coil. A stationary magnet next to a stationary wire did nothing at all. Only relative motion between the magnet and the wire produced a current, and that single observation, electromagnetic induction, became the working principle behind almost every generator on Earth.
Change is what matters, not the field itself
Faraday's law, later formalised mathematically, states that a changing magnetic field through a loop of wire induces an electromotive force — a voltage — that drives a current through the loop, and the size of that induced voltage depends on how quickly the magnetic field passing through the loop is changing, not on how strong the field is at any fixed moment. A powerful magnet sitting motionless next to a coil produces no current whatsoever, no matter how strong its field is, because nothing is changing. Move that same magnet toward or away from the coil, and a current appears, with a stronger current produced by faster motion. This is why the same underlying law also explains why a stationary current-carrying wire doesn't induce anything in a nearby wire, but a wire carrying a rapidly changing (alternating) current does — the changing current itself creates a changing magnetic field, which is enough to induce a current in a neighbouring loop even with nothing physically moving at all.
From a laboratory curiosity to nearly all the world's power
Electromagnetic induction is the operating principle behind essentially every electrical generator, from a hand-cranked dynamo to a massive turbine at a power plant: mechanical energy — falling water, expanding steam, spinning wind blades — is used to physically rotate a magnet (or a coil) relative to the other, and Faraday's law converts that continuous relative motion into a continuous electrical current. The overwhelming majority of the world's electricity, whatever its original energy source, is generated this way, all tracing back to Faraday's original, deceptively simple laboratory observation that a magnet had to be moving, not merely present, to do anything useful to a nearby wire.
What we're still unsure about
Faraday's law itself is one of Maxwell's four equations underpinning classical electromagnetism, mathematically precise and experimentally confirmed to extraordinary accuracy — it isn't a matter of ongoing dispute. The genuinely interesting open questions sit elsewhere: at extremely large scales, in the physics of astrophysical magnetic fields (like how stars and galaxies generate and sustain their own magnetic fields through a related process called the dynamo effect), the detailed behaviour remains an active area of research, since the fluid dynamics of a star's interior are vastly more complex than a laboratory wire and magnet, and predicting exactly how astrophysical magnetic fields form, strengthen, and sometimes reverse is still only partially understood.
This sits inside Electromagnetic Induction & Faraday's Law, one of eight topics in Electromagnetism, one of five domains in Physics, one of seventeen subjects the app can quiz you on.