Hotspots are widgets that are visible on your page when creating your prototype, but don't show in preview mode (unless you choose to show them in preview settings).
They let you link a particular part of an image to another page without having to chop up the uploaded image.
To use a hotspot, add one from the library and select the link button to add a link from that widget.
Hotspots are useful if you want to link from more than one area on an image. If you want to link from the whole image you can do this too with a normal link.