The Limit Distance extension prevents your target from leaving the screen area. For example, if you have a very smooth camera movement, if the player moves very fast, he might get out of the screen. By using this extension, the target will push the camera in order to always stay inside the visible area.
Limit Horizontal Distance: If enabled, the target will push the camera horizontally once it reaches the defined limits
Limit Vertical Distance: If enabled, the target will push the camera vertically once it reaches the defined limits