So we need to break this down into two parts, what are Newtons laws and what is terminal velocity? In this case we will only need the first two of Newtons laws. Number one " An object will remain at rest or move with constant velocity unless acted on by a resultant force". Number two is simply force is mass multiplied by accelaration. Terminal velocity is when there is no resultant force ( ie sum of forces is 0) acting on a falling object. When the object is initially dropped it will accelarate down because of the force of gravity (N2L) as it gains speed it collides with air particles, the faster the object the more frequent to collisions hence the greater upward force. At somepoint during the fall the upwards force from bumping into the air particles (drag) cancels with the force from gravity hence no resultant force by (N1L) it will move at constant velocity, which is terminal velocity.