Motion can be studied in different dimensions. Study of motion of a body in a straight line is called motion in one dimension. A falling body can be called as motion in one dimension. Things can also move sideways when they fall at the same time. This is also called as projectile motion and it is a study of motion in two dimensions. When studying projectile motion we can neglect one dimension of the motion and study the other dimension obtaining the results of motion in that dimension. If a body is thrown at sideways and another body dropped vertically both of they reach the ground at the same time. This is because the vertical motion of the body projected sideways can be directly compared with the dropped body ignoring the sideways motion of the first body. This video talks about studying motion in two dimensions.