Normally, the vestibular system in your inner ear detects movement with fluid and sensitive hair cells, and sends that information through nerves to your brain.
On a rocking boat, my vestibular senses say I'm moving up and down, but if I'm looking at the deck, my eyes are telling my brain that I'm sitting still.
It combines things like vestibular sense, which is your sense of your head's orientation, and proprioception, your sense of where your body is and where it's moving, along with other touch-based sensations.