Like Microsoft Windows, Ubuntu has the ability to group multiple windows of the same application into a single pane in the window list. When you do this, the application name is shown in the window list, and you can click on this to see the name of each window that is open for this application. This behavior is not on by default however. To turn it on, here's what you need to do.

Right-click the window list section of your panel and select "Preferences" from the menu that is shown.

When the window list preferences window there will be a section labeled "Window Grouping". You can select either the "Group windows when space is limited" which will, as the name implies, only group windows when there is not enough space to show the name of each open window. Or you can select the "Always group windows" option which will always make use of this feature.