This will display the cross hairs for the selected chart. The cross hairs will display the date and price on the X-axis and Y-axis for the chart. If the Data Window is active, the values will reflect where the cross hairs are located on the chart as well as any Chart Studies that are currently loaded.
The Cross hairs can be selected from the toolbar shown below, by right clicking on the chart and selecting "Cross hairs," or by choosing the "Chart" tab from the menu bar and selecting "Cross hairs."