The phrase is a metaphor, meaning it uses figurative language to create an image.
Here's a breakdown of the meaning:
* "Whose stern impassioned stress" refers to the challenges and struggles faced by America throughout its history.
* "A thoroughfare freedom beat" means that freedom is an essential part of America's identity and a path that should be open to all.
* "In America Beautiful" connects these struggles and aspirations to the beauty and promise of the nation.
In short, the line suggests that the fight for freedom is a defining characteristic of America's past and present, and that freedom is a fundamental right that should be accessible to everyone in this beautiful land.