Drag is the force that slows the aircraft down. Drag is basically friction, the resistance of the air against the structure of the aircraft. This may be a bit difficult to understand. If you put your hand out the window of a moving car, the wind pushes against it and tries to push it back. This is drag.
Aircraft designers try to eliminate as many drag inducing features as possible.
The smoother an aircraft's surface is, the less drag will be induced. But you can never completely eliminate drag. It is very interesting to note the Soviet design concept. The Soviets make the front of the aircraft as aerodynamic, thus less drag inducing, as possible. Nose, wing leading edges, anything that comes into contact with the air first is made as smooth and flawless as possible.