When a user selects an object inside the diagram (or right-clicks on an abject and selects Show Properties View from the menu) this tab is displayed. It contains a list of properties that are related to the selected object. Some of the properties are read-only, while others are editable.
Editable properties fall into several categories. Some, like the Description field, are directly editable by typing into the value column. Others allow you to select a value from a drop-down list or enter a list of values into a specific interface. Any changes to the property values will be recognized and applied to SharePoint at the time of the Update to SharePoint.