A scale constraint matches the scaling of one object to one or more other objects. This constraint is useful for making several objects scale simultaneously. For example, you can make a group of characters all look in the same direction at the same time by animating one character’s head and then constraining all the other character’s heads to the head you’ve just animated.

A constrained object is an object whose scaling is driven by the scaling of one or more target objects. The scaling of one or more target objects is called the target scale.

See Create scale constraints and Constrain > Scale.