The ideal gas law relates four key properties of a gas, its pressure, volume, temperature and the quantity of gas present, within a single equation. Because these four properties are mathematically linked rather than independent of each other, the ideal gas law lets a physicist or engineer predict exactly how changing any one of them, compressing a gas into a smaller volume, or heating it, will affect the others, as long as the gas in question behaves closely enough to the idealised model the law describes.
The four variables are locked together, not free to change independently
A gas's pressure, volume, temperature and quantity can't all change independently of each other while the gas continues to obey the ideal gas relationship; fixing three of the four variables automatically determines the fourth. This means, for instance, that if a fixed quantity of gas is compressed into a smaller volume while its temperature is held constant, its pressure must increase in a precisely predictable way, and if that same gas is instead heated while its volume is held fixed, its pressure must rise in an equally predictable, calculable manner. The ideal gas law's real practical power is exactly this: it turns what might otherwise seem like several separate physical relationships into one single, unified mathematical statement.
"Ideal" describes a simplifying model, not every gas exactly as it really behaves
The law is called the ideal gas law because it describes an idealised model of gas behaviour, one that assumes gas particles have negligible volume themselves and don't meaningfully interact with each other beyond simple collisions, an approximation that works remarkably well for many real gases under a fairly wide range of everyday temperatures and pressures. Real gases deviate from this idealised model most noticeably at very high pressures or very low temperatures, conditions where gas particles' own actual volume and their attractive forces on each other become significant enough that the simplifying assumptions behind the ideal gas law start to break down.
What we're still unsure about
The ideal gas law itself, and the specific conditions under which real gases deviate from it, are precisely characterised, extremely well established classical physics and chemistry, confirmed across an enormous range of laboratory and industrial measurement. What's more genuinely an ongoing area of applied research is developing increasingly accurate correction models for gas behaviour under especially extreme conditions, very high pressure industrial processes or cryogenic applications, where even the more sophisticated correction models physicists have already developed beyond the simple ideal gas law still leave some genuine, measurable prediction error — researchers continue refining these more advanced models for the hardest, most extreme real-world cases, rather than treating gas behaviour prediction as a fully closed problem at every possible condition.
This sits inside The Ideal Gas Law, one of seven topics in Thermodynamics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.