render json:

dogpark has been HTML this whole time

Every controller action you've written so far — the scaffolded Walker views back in an earlier module — renders an .html.erb template. A browser asks for /walkers, Rails hands back a full HTML page. That's the ONLY thing a browser can display directly, so it made sense while you were learning Rails' shape.

But PawWalk's real clients are iOS and (later) Android apps. They don't want a page — they want data: a JSON object they can decode into a Walker struct. Rails controllers don't care what format they respond in; render just needs a different argument.