Hay represents two-dimensional motion

If a body is moving from left to right or right to left it is linear motion in x- direction. It is also called one dimensional motion.

If a body is moving up to down or down to up it is linear motion in y- direction. It is also called one dimensional motion.

But if a body is moving forward as well as downward it travels in X direction and in y direction at the same time. Such a motion is called two dimensional motion.

E.g motion of a ball.

Attachment shows path of two dimensional motion of a ball.