Prop walk is a predictable sideways force when in reverse. It can be useful when anticipated, but it pushes the stern to one side whether you like it or not, and can be a problem in tight spaces. Different props "walk" differently, and rotation determines the vector of the pull. Physics from wiki:
Propeller walk is caused by the water, moved by the propeller in an axial direction and in a rotation. The water, coming from the propeller, gets a cone shape, widening when it leaves the propeller. If the rotating water cone contacts the ship's hull, a sideways force is generated. Propeller walk is hardly noticeable when sailing forward, since the propeller water will not hit a large surface of the ship's hull and corrections to the ship's course can easily be made with the rudder. When in reverse gear, the water will hit the hull directly, resulting in propeller walk.