This element is used to align content to the horizontal center of the content area via a block level element (div). This is to be used if content does not recognise text centering (i.e.: the Centered (p) formatter).
(Click image to enlarge)
Highlight the content you want to be affected and then select the element from the right-hand list. The code will be placed in the content field.
You can also use this Toolbar button as a shortcut:
Centered (div) code appears like this: