Free Transform plane
 
 
 

A yellow square appears at the center of the Transformation planes when you are in the Selection or XYZ Selection mode. This is the Free Transform plane.

Sphere in Scaling mode A. Free Transform plane

Dragging the Free Transform plane in Translation or Scaling mode lets you move or scale an object in any direction.

In Rotation mode, the Free Transform Plane becomes a yellow Free Rotation ring so you can rotate your object in any direction.