The Linear controller interpolates between animation keys by evenly dividing the change from one key value to the next by the amount of time between the keys.
Linear controllers do not display a properties dialog. The only information stored in a linear key are the time and animation value.
Use Linear controllers whenever you want a very regular, even transition from one key to the next. For example:
No properties dialog is available for linear controllers.
Change linear key time by moving keys horizontally in Track View Edit Keys or Function Curves mode.
Change linear animation values by moving keys vertically in Track View Function Curves mode or by directly changing the parameter value in the viewport with the Animate button on.
The animated object has a mechanical motion.
You can also assign this controller in Track View.