The models in question are Pixel 2 and 3. Recorder app was originally incompatible with any Pixel phones other than this year’s Pixel 4, but Google has patched it. The owners of Pixel 2 and 3 can now download and install it via Play Store.
The function of Google Recorder is to record speech and then transcribe it into text – a useful function for journalists or geniuses with insomnia. The app does not work perfectly, having issues understanding mumbled phrases and being unable to tell different speakers apart, but hopefully Google will polish it over time.