Hotspots are active areas on your page which are not a visible object but still respond to user-interaction. Essentially they allow you to make part of an object or image interactive.
This lets you make part of an image or object into a hyperlink which is ideal for letting your user click for details of specific parts of an object - a map for example. Hotspots also let you cover several objects with an active area which might be used to make the edge of a page active irrespective of what objects were on the page.
Any of the Opus triggers can be used with a hotspot object so you can even make a hotspot respond to the mouse moving over the area. Examples of uses for this might be to provide a label for that area or instructions on what will happen when you click which only appear when you move the mouse over them and thereby reduce screen clutter.
Hotspots are drawn using the Opus drawing tools and so can be drawn freehand, or using regular objects such as squares, circles and stars. You can set the outline and fill colour for the hotspot so you can distinguish it in the editor, though the user will not see these colours. Bear in mind that if the hotspot has no fill, only the outline is active and a thin outline can be tricky to click on when you can't see it!
Opus Pro and Opus Creator Reference - Creating and Using Hotspots