Whiteout is a weather condition that causes disorientation and low visibility by snow, overcast cloud and fog. Basically, the whiteout in aviation occurs when the pilots cannot see the visible horizon because of the terrain covered with snow in the white sky. Also, blowing snow may lead to the whiteout due to reduced visibility.
Imagine, if you can, landing a helicopter in a place where the sky and ground are the same colour and there are no shadows to show the contours of the surface that you are landing on or to even show you how far away the ground is!
Distances, depth, contours and focus can be heavily impaired in whiteout conditions. As piloting can be very challenging in these types of situations, it is best practice to avoid flying in whiteout conditions.
Check out the Transport Canada AIM section 2.12.7