What does the term drag mean in flight?

Drag is one of the four forces applicable to a flying aircraft: thrust, drag, lift and weight. Drag is the amount of resistance to forward motion caused by a solid body moving through a fluid (air is considered a fluid for the purpose of aerodynamics).

The easiest way to feel the effect of drag is to stick one's arm out a car window while it's moving. First do it palm-down (horizontal), and then palm-forward (vertical). The force, known as "drag", pushing the arm backward increases substantially when the palm is vertical. That force is the same effect that an aircraft in flight experiences due to the frontal area of its airframe forcing its way through the air.

The science of aerodynamics seeks to minimize the drag through streamlining and other design factors, since drag wastes fuel, but it is physically impossible to elimnate it.