The terminal velocity is the velocity a body will reach when it is falling to the surface of the Earth. Sometimes a body will not reach this velocity as it will keep accelerating until it hits the Earth.
As a body moves towards Earth, it experiences two forces upon it, weight and air resistance. As the body starts to move towards Earth the velocity is low and therefore there is minimal air resistance. The body is in constant acceleration at gm/s2. As the velocity increases, the air resistance also does until the forces acting are equal and it is at its terminal velocity.