HTML Overlay Widgets allow you to show free-form HTML within the Overlay. These are particularly useful when you want to show a very basic display of something that requires a bit more customization than one of the standard types, but does not rely on outside information from events. This can be thought of as a more basic version of the Custom Overlay Widget, as there are no pre-defined functions to connect data up to.
The HTML Overlay Widget can be set to automatically refresh the contents shown via the "Refresh Time" option on the Details tab.