Adverse yaw is a secondary effect of the ailerons. When the ailerons are actuated, one of its effects is an asymmetrical change in drag. In essence, when the ailerons are being used, drag is increased on both wings to a certain extent, but this drag is stronger on one side than the other.
The result is that when rolling an aircraft for a turn, the aircraft will have a tendency to want to yaw in the opposite direction of the roll.