What is teminal velocity?

A freely falling object in air (or any other fluid for that matter) experiences two forces: the gravitational force and the force due to air resistance (or drag force).
While the gravitational force depends only on the mass of the object and acceleration due to gravity, for the most part we can consider it remains constant. The drag force on the other hand is a function of the velocity of the object, i.e. the magnitude of this force increases with increase in velocity.
When the object starts to fall the drag force is zero, this means that it only experiences one force that is the gravitational force that pulls it downwards. As the object gains velocity, the drag force gains magnitude and begins to act in the upward direction (i.e. in the direction opposite to the motion). This means that the net force experienced by the object reduces.
Eventually the object attains a velocity such that the drag force is equal in magnitude to the gravitational force, thus making the net force experience by the object zero.
This velocity is known as "terminal" velocity, since a freely falling object cannot fall faster than this.