Our brains rely on our other senses and try to make sense of conflicting signals at times. It usually does a very good job. We stand upright and turn our heads without falling over, whether our eyes are open or shut.
Our inner ear senses movement and our position relative to the earth: acceleration and what way is up, which is the acceleration from gravitational forces.
The system detects movement: linear movement along each axis, and rotational direction around that axis, by detecting movement in fluid in tiny canals oriented to each axis. If the movement stays constant, the fluid does too, so acceleration is really what is detected.
At the same time our brain is busy comparing what this system detects with what we see, and feel. Sometimes it gets things wrong, and we get that sensation of movement even if we aren't because it gets images that tell it we are moving relative to something we see.