What does your touch receptors actually sense?

Like, your optical receptors sense/measure the frequency of waves of light and send that signal to the brain, your auditory receptors also measures the frequency of a wave, a sound wave, and sends that signal to the brain. Taste and smell I guess just react to specific molecules chemically. But what about "touch receptors"? What actual quantities do they measure and send to the brain? Mass? Speed? Acceleration? Displacement? A combination? Force? I don't know why but it seems so unintuitive to me either way for some reason. Or am I just thinking about this whole thing the wrong way?