For atomic particles (protons, neutrons, electrons), their average velocity (almost never the same across a given sample, and less likely the larger the sample) is measurable, and we call that temperature. It’s a way to describe the total energy of a system. As it happens, we use that property for everything from steam turbines to cutting torches to deep freezers. It’s a simplified answer, but the engineering realities take into account many other properties and variables which relate to temperature in order to achieve the desired effects.