Aerodynamics and drag
Any body that moves through a gas or a liquid creates drag. Drag causes a constant dissipation of the energy that the moving body has, which means the moving body has to constantly expend energy in order to keep going. Only in a complete vacuum, e.g. space, does a body that has been set in motion retain its velocity without further expenditure of energy. Of course space is otherwise very inhospitable, so organisms have evolved and machines been designed in order to cope with the realities of drag in a gas (e.g. our atmosphere) or a liquid (e.g. the water in our oceans).
Fish and marine mammals have evolved torpedo or droplet like shapes, with ultra smooth skin. Fast moving land animals have evolved limbs with the main muscles attached high on the limb to reduce the turbulence they create as they move at high speed. Birds have streamlined bodies and retract their feet when flying, all of which reduces air drag. Cars, once box/brick like looking vehicles have over the decades been shaped by designers to be more an more aerodynamic, with dramatic reductions in fuel consumption as a result. Submarines and airplanes are not brick shaped craft with myriads of external attachments, but sleek and smooth somewhat drop-shaped machines that can slice through the water/air at high speed.
The laws of physics have it that the drag created by a body moving through a gas or liquid increases with the square of the velocity with which that body moves. Put it in a different way, going from A to B in a liquid or gas at 100 mph / 160 kph (in an ideal lab scenario and all else being equal) requires not the same, not twice but four times as much energy as going at 50 mph. Since power is a measure of energy per unit time, and the time available to output the required energy at 100 mph is only half that when moving at 50 mph, the power required to move twice as fast adds another factor of 2. So it takes 8 times as much power to move at 100 mph than is required to move at 50 mph. Building and installing an engine with 8 times more power or somehow attaching muscles to an animal to make it 8 times as strong is a huge problem, it adds weight, it requires more bulk, it is more costly. This is why aerodynamic streamlining is so closely connected to moving at high speed, because it increases speed by saving energy, rather than by adding more power.
The air drag acting on an aerial vehicle has different sources. A major source is form drag. Pulling a brick through the air is far harder than pulling a drop shaped sleek body of the exact same size. A brick moving through the air creates a huge amount of energy expensive turbulence, while an ideal drop or egg shaped, streamlined body, leaves the air through which it moves almost undisturbed.
Another major source of airdrag is skin friction, which describes how energy is lost due to air molecules 'rubbing' against the skin of a body moving through the air. The less total area a body moving through the air has, the less the air friction. Highly aerodynamic, streamlined bodies, typically experience the vast majority of their airdrag coming from skin friction as the form drag component has been largely eliminated due to the streamlining.
The Cd valued of an airplane is measured experimentally in a wind tunnel and describes its aerodynamic efficiency. A value of 0.1 indicates a more aerodynamically efficient airplane, than a value of 0.2. However, this value is typically calculated in relationship to the wing area of the aircraft. When multiplying the Cd value of an airplane with its wing area one arrives at what is called the Drag Area. The Drag Area roughly describes the effective air resistance the aircraft experiences AS IF the airplane was a flat, square plate with area Cd x wing area, dragged through the air.