With React Native, instead of writing HTML/CSS and running it on a crappy browser embedded in an app, you write code that looks like HTML/CSS (it's actually JSX)[1][2] and it is translated to native code.
It's not translated. The JS runs to create a declarative description of what the ui should look like, which is reduced to a diff representing the changes that needs to be made to the native ui. These change commands are then marshalled to native code which simply implements the changes (add this thing, move this thing, remove this thing).
[1] https://facebook.github.io/react/docs/displaying-data.html
[2] https://facebook.github.io/react/docs/jsx-in-depth.html