The Rotation constraint transfers rotation between two objects. This constraint uses the position of a source object or objects to constrain the rotation of another object.
For example, in the following pictures, one cube (source) rotates and the other cube (constrained object) rotates with it.
Rotation constraint A. Source object B. Constrained object
The constraint source rotates with the constrained object.
When the Source (A) is rotated, the constrained object (B) rotates with it.