Animating a parameter refers to changing its value over time. You do so by setting keyframes (frames at which you explicitly define a value) and allowing Nuke to interpolate the values in between. You can animate most of Nuke’s parameters in this manner.
Working with Animated Parameters
Animated Parameters in the Dope Sheet and the Curve Editor
Working with Keyframes in the Dope Sheet
Using the Dope Sheet Interface
Viewing Keyframes in the Dope Sheet
Selecting and Editing Keyframes in the Dope Sheet
Editing Clips in the Dope Sheet
Synchronizing the Frame Range Between the Dope Sheet and the Curve Editor