Without air, all objects fall with the same acceleration and if dropped from the same height, will hit the ground at the same time.

When there is air, this is not the case, and objects that are heavier, and more aerodynamic will hit the ground before objects which are lighter and less aerodynamic.

Above, you can see a hammer and a piece of paper being dropped from the same height. The hammer will hit the ground first as it is heavier and more aerodynamic.

Below, you can see this happening as the camera catches the hammer accelerating ahead of the paper.

As predicted in air, the hammer which is heavier and more aerodynamic hit the ground before the paper.