In flight, the static pressure above the wing is lower than the static pressure below the wing. A component of the airflow moves sideways along the wing from the fuselage to the wing tip. This span-wise, flow spills up around the wing tip to where the lower pressure is and flows back towards the fuselage on the upper surface of the wing.

At the wing tips, the span-wise flow is the strongest. Strong vortices form here, at the trailing edge of the wing. These are known as wingtip vortices. And they look something like this. These vortices require extra energy to generate. And this is bad for the aeroplane's efficiency. These vortices also contribute to drag.

And here is how it works: The upward flow of the vortex is outside the span of the wing, but the downward flow is inside the span, behind the wing. The downward flow of the vortex deflects the relative airflow further down, This is called downwash.

The deflection of the airflow downwards causes the wing to experience a local airflow. The direction of the local airflow is the average between the free stream relative airflow and the direction of the downwash. The aerodynamic force produced by the wing acts at a right angle to the local airflow.

But since the local airflow is inclined downwards, the aerodynamic force is inclined backwards by the same amount. A component of the lifting force acts parallel to the free stream relative airflow and opposite to the direction of flight. This component acts in the same direction as the direction of drag. This component is undesirable, unavoidable, and it is known as induced drag.