These nodes correspond to the native “Constant” nodes.

However, they have optional evaluation modes controlling how their values are retrieved if they are animated via an envelope:

• Per Frame
  Just like with the native Constant nodes, the value will be retrieved once per frame
• Interpolated
  This will retrieve the value for the current and next frame and interpolate between them in the case of motion blur
• Per Sample
  This fully evaluate the envelope every time and may be quite slow

You can use the output of these nodes to, for example, change the position of a texture and have that change in position be motion blurred.

Interpolated is the preferred method to use, Per Sample should only be used if the result of Interpolated isn't accurate enough.