Netflix: what does it mean?

The orange underlined, can someone explain that to me?

This relates to the sound signal.

Dolby 5.1

https://www.heimkinoraum.de/tests/dolby-surround-systeme-8211%3B-von-den-anfaengen-bis-dolby-atmos-54

In short: 5 speakers distributed around the room, plus a subwoofer

5.1 (pronounced "five point one"), more precisely surround sound 5.1, denotes

https://de.wikipedia.org/...-Tonsystem, in which five main channels and a separate bass effects channel are available.

https://de.wikipedia.org/...g/wiki/5.1

This indicates that there are five main channels for sound and one channel for low frequency effects (bass).

https://de.wikipedia.org/...g/wiki/5.1