The Properties pane shows type specific information about selected items. When an item (e.g. point line etc.) is selected in a photo table or 3D Viewer, the property sheet of the selected item is displayed, which shows data like Material, Layer, ID number etc. When multiple types of items are selected, the Properties pane will show common properties as well as the data specific to the items selected. When a cell is clicked, a description of that property is shown at the bottom of the pane.
See The Objects That Make Up a Model for details on the information displayed on this pane when certain items are selected.