This control was initially developed by David Grieve before being integrated into ControlsFX.
It is designed to show a small blurb of text above a node (most commonly an ImageView, but it will work with any Node). The text can be collapsed down to a single line, or expanded to show the entire text.
In some ways, it can be thought of as an always visible tooltip. By default, it is collapsed so only the first line is shown – hovering over it (or clicking on it if the showOnHover functionality is disabled) will expand it to show all text.
Shown below is a screenshot of the InfoOverlay control in both its collapsed and expanded states: