It means that horizontal and vertical motions are independent to each other, which means they don't influence one another.
Motion in physics is defined to be a change in position of an object over time. We can describe motion in terms of displacement/distance, time, velocity/speed, acceleration and the like.
There is one-dimensional motion and there is also two-dimensional motion. In two-dimensional motion, there are horizontal motion and vertical motion. Horizontal motion is a projectile motion in a horizontal plane. Meanwhile, vertical motion is a projectile motion in a vertical plane. Both motions are independent, meaning that they don't actually influence each other.
Learn more about motion here: https://brainly.com/question/11213880
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