See Assigning Controllers for steps on how to assign a controller in the Motion Panel or Animation menu.
Animation controllers and constraints provide powerful tools for animating all the objects and materials in a scene. For example, rather than keyframing the position of an object in your scene, the object can follow a spline using the Path constraint, react to any animated parameter using a Reaction controller, or move to a musical beat using the Audio controller. You can use a List controller to combine controllers with individual weighting. You can drive a single vertex or control point on a complex object by a variety of controllers.
Controllers: Track View to display the icons for Assign, Copy, Paste. Delete Controller and Make Controller Unique.

To view lists of available controllers and constraints, see Animation Controllers and Animation Constraints.
Technically, there is no difference between a controller and a constraint. A constraint is simply a controller that requires the use of a second object. For example, a Path constraint is a controller that requires a spline object for a path.
Special-case controllers are not assigned manually with the Assign Controller command. They are applied automatically during certain procedures:
Geometry
Compound Objects
Morph.
Global Tracks. A Driven controller transfers key data to a Block controller. See
Block Controller.

Assigning a controller in Track View
Example: To assign an animation controller in Track View:
Controller submenu, choose Assign.

Assign Controller dialog
If a parameter has already been animated, then assigning a new controller has one of the following effects: