Proper motions refer to the apparent motion of celestial objects across the sky over time, as observed from Earth. This motion is caused by the actual movement of the object through space, rather than just the Earth's rotation. Proper motions are typically measured in units of arcseconds per year and provide valuable information about the dynamics and evolution of galaxies, star clusters, and individual stars. By studying proper motions, astronomers can better understand the orbits, ages, and interactions of celestial objects within our universe.