By default, Meshmixer uses a Perspective Projection, so things in the distance get smaller. This is more natural for viewing 3D models, and tends to agree with our intuitions about 3D space. However, in many technical CAD programs, an Orthographic Projection is used instead, which does not distort angles. On the View menu there is a checkable item for toggling Orthographic View on or off.
This command shifts the camera so that the view is centered at the point under the cursor, but only if the cursor is over an object in the scene. When doing a Tumble operation, the center-point of the rotation will be this new target point. This is very helpful when sculpting! Also, the 'c' hotkey is a quick way to pan around the scene.
This pair of hotkeys is basically "undo" and "redo" specifically for camera movements. Each time you move the camera, it is saved on the Camera stack, and these keys allow you to browse previous viewpoints.