The overlay resource type lets you take an image, position it on a map (GPS location, size, rotation) and will have location questions in the form overlay the image on their maps.
Below is a screenshot from the Resources section creating an overlay resource:
This image is then overlayed in the mobile app when the user is viewing or capturing the location:
In the mobile apps you're able to toggle between map and satellite, adjust the opacity of the overlay (or turn it off), as well as view your current location relative to it.
If a location question with an overlay is also the primary question in a repeat group, the overlay will be included in the repeat group UI showing the pins of the entries within that group.
Overlay resources can be created via the Resources API, and location questions in forms and dispatches can include the overlay resource when they're being created via the API.
When using the API, the overlay resource is a JSON file that follows the KML conventions for orienting an image onto a map: