Adds a keyframe to all curves (X, Y, Z) for each key that is encountered . For example, if a light has a keyframe set on its X translation function curve, a keyframe is set on its Y and Z translation curves at exactly the same time. This synchronizes all keyframes on each of the object’s function curves.
If you do not have all three (X, Y, and Z) of the attribute’s curves selected, Key Sync has no effect. 
Key synchronizing A. Original curve B. Key Sync applied