Audio description is an "optional narration that describes what is happening on-screen, including physical actions, facial expressions, costumes, settings and scene changes," per Netflix. You can enable audio narration by clicking the Dialog icon, though it's not available for all titles.
In the app, see which titles have audio description available by clicking Search and scrolling to the very bottom. On desktop, find titles with audio description here or scroll to the bottom of the homepage and click Audio Description.
iPhone users can also listen to the audio description with the screen off. First, play the title with audio description enabled. Next, turn the screen off; this will also turn off Netflix. Then, turn the screen back on and press the play button.