Hotspot or invisible links are found in the uploads library at the bottom under all of your own uploads.
Hotspots are links 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.
Usually you would use a button widget to link to the next page, here you will be using an image of a button with an invisible link area over it. This will link to the next page.
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.