Here’s a little experiment for the willing to try:
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Hold your arm out straight, pointing your index finger into the air with your nail at eye level. Start with it to the far left, and slowly move it laterally to the right, tracking it with your eyes. Don’t move your head, only your eyes. They roll smoothly, since you have something to fix your attention to.
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Try this without your finger, having nothing to follow. Track your eyes across your field of view, side to side. Your eyes will move in “steps”, unable to pan as smoothly while tracking something.
I’ve always thought this must help describe something relevant to our vision capabilities and how such a simple motor skill actually becomes difficult, once you try it. I can’t even force my eyes to track smoothly - almost, though.