Rust forming slowly on an iron nail, a battery driving current through a circuit, and a match bursting into flame look like three unrelated phenomena. Chemically, they're the same basic category of reaction: oxidation-reduction, or redox, in which electrons transfer from one chemical species to another. Whatever else is happening in a redox reaction, one absolute rule never breaks — every electron one atom loses is an electron some other atom gains, with none created or destroyed along the way.
Two halves of the same transfer, always paired
Oxidation is the loss of electrons by a chemical species; reduction is the gain of electrons by another. The two always happen together in the same reaction, which is why chemists conventionally split a redox reaction into two "half-reactions" for analysis — one showing the oxidation, one showing the reduction — even though neither can actually occur alone in isolation; there's no such thing as a species losing electrons without some other species simultaneously accepting them. In rust formation, iron atoms lose electrons (are oxidised) while oxygen atoms gain them (are reduced), forming iron oxide. In combustion, the fuel's atoms are oxidised while oxygen is reduced, releasing energy as heat and light in the process.
Why this makes batteries possible at all
A battery exploits redox chemistry's electron transfer directly and deliberately: rather than letting oxidation and reduction happen at the same physical location, as they do in a rusting nail, a battery physically separates the two half-reactions into different compartments, forcing the electrons being transferred to travel through an external wire to get from the oxidation site to the reduction site — and that forced detour through an external circuit is exactly what produces a usable electric current. The battery's voltage reflects how strongly the reduction half-reaction "pulls" electrons relative to how readily the oxidation half-reaction "releases" them, a difference chemists quantify using standard electrode potentials, which let you predict, before ever building anything, roughly how much voltage a given combination of chemical half-reactions will actually produce.
What we're still unsure about
The fundamental principle of paired electron transfer in redox reactions is completely settled chemistry, confirmed across an enormous range of reactions and applications — it isn't in dispute. What remains a genuinely active area of applied research is engineering redox reactions for specific practical purposes with better performance — designing battery chemistries with higher energy density, longer cycle life, and lower cost is fundamentally a problem of finding and optimising the right redox half-reactions and materials, and despite redox chemistry's underlying principles being fully understood, discovering genuinely better combinations for large-scale energy storage remains a significant, only partially solved materials science and chemistry challenge.
This sits inside Oxidation-Reduction Reactions, one of eight topics in General Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.