It runs JS in the target platform's JS engine (iirc in a background thread) and renders the UI using the target platform's components. This gives it the flexibility and development speed of a webapp, but the UI performance of a native app. In theory anyway.

Compare also with Flutter [0], a new competing application development stack from Google with a similar architecture (= components all the way down) and development experience (= live reloads), but which will in production compile everything to native (instead of interpreting JS).

[0] https://flutter.io/