Kinetic theory explains a gas's measurable macroscopic properties, pressure and temperature especially, entirely in terms of the constant, random motion of its individual, invisible molecules. Pressure, in this picture, is simply the accumulated force of enormous numbers of molecular collisions striking a container's walls every second, and temperature is a direct measure of how fast, on average, those same molecules are actually moving, not some separate, independent property a gas happens to have.
Pressure is a statistical consequence of countless individual molecular collisions
Kinetic theory treats gas molecules as being in constant, random motion, colliding with each other and with a container's walls, and models those collisions as perfectly elastic, no kinetic energy is lost overall when molecules collide. A gas's pressure isn't some separate substance-like property it independently possesses, it's the statistical, large-scale result of how often, and how forcefully, an enormous number of individual molecular collisions strike the container's walls each second, a genuinely mechanical explanation for what pressure actually is.
Temperature turns out to directly measure average molecular kinetic energy
A gas's absolute temperature is directly proportional to its molecules' average kinetic energy, which is exactly what explains why heating a gas held at constant volume raises its pressure, faster-moving molecules hit the container's walls both harder and more often. That same connection links kinetic theory directly back to the macroscopic ideal gas law, the ideal gas law turns out to be the large-scale statistical consequence of this underlying microscopic picture of countless individual molecules in constant random motion, rather than a separate, independently discovered relationship.
What we're still unsure about
That kinetic theory explains pressure and temperature in terms of molecular motion and collisions is well established, extensively confirmed physics. What's more genuinely a known, quantifiable limitation is that kinetic theory's simplifying assumptions, treating molecules as point-like with no actual volume, treating collisions as perfectly elastic, ignoring intermolecular forces except during a collision itself, are genuine approximations describing an idealised gas closely but not exactly, real gases deviate from those predictions more noticeably at high pressure or low temperature, where actual molecular size and intermolecular attraction start mattering, and exactly how large a correction a specific real gas needs under specific conditions remains a matter of ongoing measurement rather than one universal fix.
This sits inside Kinetic Theory of Gases, one of seven topics in Thermodynamics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.