In his third paradox, Zenon states that for motion to be occurring, an object must change the position which it occupies.
He gives an example of an arrow in flight. In any one instant of time, for the arrow to be moving it must either move to where it is, or it must move to where it is not. It cannot move to where it is not, because this is a single instant, and it cannot move to where it is because it is already there.
In other words, in any instant of time there is no motion occurring, because an instant is a snapshot. Therefore, if it cannot move in a single instant it cannot move in any instant, making any motion impossible.
Imagine we could take a photo of every instant of the movement of the flying arrow. In every picture the arrow would be stopped, not moving.