Refer to figure.
Adverse aileron yaw is an unintended yaw which occurs when the aileron are used. It is due to the difference in induced drag between the wings. The wing with the down-going aileron has higher induced drag, which causes yaw in the opposite direction to the commanded roll. 

So if using the ailerons to roll left, there will be an initial yaw to the right.

Induced drag occurs because of wingtip vortices. The flow of higher pressure air from below the wing to the lower pressure air above the wing creates these vortices. The vortices change the direction of airflow over the wing, which tilts the lift vector backwards slightly, creating induced drag. The more lift a wing produces, the more induced drag it creates. 

• Left roll → the right wing produces more lift and more induced drag→ adverse aileron yaw to the right
• Right roll → the left wing produces more lift and more induced drag→ adverse aileron yaw to the left

It can be minimised by using differential ailerons or Frise ailerons.