Constraints are object features, they control in various fashions the object's transform properties such as its position, rotation and scale. Use this tag for questions pertaining to constraints and their usage within Blender.
Constraints are a way to control an object’s properties (e.g. its location, rotation, scale), using either plain static values (like the “limit” ones), or another object, called “target” (like e.g. the “copy” ones).