https://tts.langeek.co/read?text=YW55&hash=ad029144eef6fdf6a676f173355656b642d85687&lang=en&cache-only=0&driver=googletts&accent=US

https://tts.langeek.co/read?text=YW55&hash=57a132618368f8dcb743b8201b9b6fc23947eb88&lang=en&cache-only=0&driver=googletts&accent=GB

These forms are used in in non-positive sentences to refer to specific things or persons without specifying exactly who or what they are.