If a skydiver jumped from a plane then the pull of gravity would cause them to go faster and faster as they fall. At the same time, air resistance would create a slowing force called drag. The faster the skydiver fell, the more drag they would experience. At some point their speeding-up force (gravity) and their resistance force (drag) would be exactly equal and balance each other out. This would mean that they cannot gain any more speed and this is known as terminal velocity.
Terminal velocity also applies to vehicles such as aircraft, trains and cars. There is no point having a really powerful engine if the drag is going to stop it from propelling the vehicle, hence why aerodynamics is a major design consideration.
Answered by Reece Hill, Science Museum.