Its the same old thing that was used in the ZX Spectrum era. You download programs / games from the tape. Only sound quality and filters are probably better.
In my opinion, better than Bluetooth or WiFi. You can also send files that you can put on some storage, and then upload them. I find these methods much simpler than sound, because if there is a lot of noise around you, you cannot do much.
It is just converting data into audio, and then back from audio to data.
Google's search for Zotyocopy and Copy86M are utilities used to save a game to tape after loading it into memory using the zx spectrum.