This button is used to add or take away metrics from the layout.
Once you click the button you will see a popup with the available pool of metrics on the left side and the metrics that are already displayed on the layout on the right side.
Simply click on the metric to move it from one side or the other. Use the filter to help narrow the search and find the metric you want to add to the layout. You can use this feature to both add and take away metrics from a layout. Once you have the metrics you want for this layout click change metrics at the bottom.