Tracks the camera forward and backward. You can also press + (or + ). Use + + to drag a marquee around the area you want to dolly in on.
View > Camera Tools > Dolly Tool >
If Center of Interest is on, -drag in the camera’s view to move the camera’s center of interest toward or away from the camera. If off, drag in the camera’s view to move the camera toward or away from its center of interest. You can + + -marquee a region to snap the center of interest to the center of that region.
Center of Interest is off by default.
If Center of Interest (and/or Local) and Bounding box are on, and you are currently in a four panel view, then when you -drag in any of the views, a red line with a small x at the end appears to indicate the Center of Interest.