Styles capture the various graphic aspects of elements such as font, fill, stroke, shadow etc. Most templates come with a variety of styles from which to choose.
If you don't find a style that suits your needs, you can modify an existing style.
You'll notice the selected element updating to reflect your changes. The selected style is overlaid with a pencil to indicate that you've changed some aspect of the style for this element.
New: Creates a new style from the combination of the original style and your changes.
Update: Updates the style to reflect your changes. All elements in the document that have that style are updated.
Revert: The changes you've made are discarded and the element goes back to having the unmodified style.