The Preview button will display a quick, low-density point cloud in an small interactive 3D Viewer window so that you can assess whether the selected settings and photos produce a reasonable DSM model. Previewing helps avoid generating and waiting for inappropriately set up DSM’s.
If settings and photos are appropriate a reasonable approximate point cloud will be displayed, though you may need to rotate the preview model around to see the approximate shape.
If a very noisy or, conversely, a very sparse point cloud is displayed, you’ll want to revisit your settings. See DSM Troubleshooting.