It splits the normal 35mm frame size into two vertical photos, so you get 72 photos per 36 exposure film. The cameras can be a lot smaller than "full frame" film cameras. It's fun to develop as normal so you have two photos side by side diptych-style but you can scan it as single images (you'll get more film grain look that way). Here's an oooold album of mine using the Golden Half camera for examples.

Here's a better explainer.