When you open the Activities overview, in the left side you can see the dash, a dock bar for your favorite applications and to offer a place for your running applications.
Every icon in the dash can be removed or moved and of course you can also add new icons in the dash. It’s usage is pretty straight, just to click upon the icon and the corresponding application will be started.
When an application is running, its icon on the dash will present a light white glow (in the left picture you could see it under the Firefox and Shotwell icons). Whenever you click the icon of a running application the existing window will be moved in the foreground and made visible again.
In contrast to the old GNOME 2 panel, clicking the icon of an application already visible it will not hide or minimize the visible window, it will simply show it again and close the activities overview.
In the application picker you can right click an application icon and choose Add to Favorites to add the shortcut to the dash so that it will be always visible for easy access in the future. The size of the icons will automatically reduce to try to make fit all the shortcuts in the dash.
Right clicking the shortcut on the dash and selecting Remove from Favorites you can remove the shortcut from the dash to gain a bit of space.
Immediately after adding or removing an icon from the favorites the message tray will offer the choice to cancel the action just made by simply clicking the Undo button.
If you right click upon an icon of a running application the option New Window will allow you to open the same application again, even if it is already running. Note that not every programs support multiple instances of the same application.
The shortcut icons on the dash can be sorted by dragging them to obtain the preferred order. This works with both favorites and temporary icons of the running applications.
During the drag operation a small white light will appear in the place where the icon will be moved if you release it.
The trashcan at the bottom of the dash will allow you to remove the shortcut from the bar. You have just to drag the unwanted icon on the trashcan icon and it will be deleted as expected.