The Camera Correction modifier applies two-point perspective to a camera view. By default, camera views use three-point perspective, in which vertical lines appear to converge with height. In two-point perspective, vertical lines remain vertical.
The amount of correction you need to use depends on how steeply the camera tilts. For example, a camera that looks up from ground level to the top of a high building will need more correction than a camera that looks toward the horizon.
Normal camera view (left) and corrected camera view (right)
Setting Direction greater than 90.0 biases the correction to the left. Setting it less than 90.0 biases it to the right.