AirPlay is a really cool technology that is most often used to send audio or video from your iPhone or iPad to AppleTV. Virtually any content (unless explicitly restricted by an app developer, ahem Comcast On-Demand) can be streamed to an AirPlay-enabled device, including AppleTV, AirPort Express, and many third-party speakers. This includes apps such as HBO Go, Amazon Video, Pandora, Spotify, NPR, etc.
One little-known use of AirPlay is sending your Mac's audio to an AirPlay device. It's easy to send your iTunes music directly within the iTunes program, but your computer can send any audio via AirPlay. On current macOS, connect your Mac and an AirPlay-compatible speaker or Apple TV to the same network. Open Control Center in the menu bar, click AirPlay under Sound, and choose the device. You can also choose Apple menu > System Settings > Sound > Output and select an available AirPlay device. Control Center sends audio to one AirPlay speaker at a time; the Music app can select multiple speakers. The screenshot shows an older macOS interface.
Now you can send the audio from Spotify, any radio streaming website, etc.