Complex sentence: PhantomJS is practically a headless web browser that contains a javascript API, but it is headless, which means that has no window nor viewport, its practically a command line where you feed javascript-bases programs in order to test your functionality.
Split into simpler sentences: PhantomJS is practically a headless web browser that contains a javascript API. Headless means that has no window nor viewport, its practically a command line where you feed javascript-bases programs in order to test your functionality.