The NETTV key is for viewing digital TV, streamed over the internet.
If your original remote control already had a key for this, for example, many newer TVs have a Netflix key, then this key will perform the same as the key on your original remote.
However, if you did not have a key like this, but still had the ability to watch astreaming service, you can use this key as a shortcut by programming a macro on it.
Most AV devices which are connected to the internet, such as Smart TVs or streaming Set Top Boxes have one or more applications they can run to access TV viewing via the internet.
Examples of these would be Netflix, YouTube, Amazon Prime, BBC iPlayer or similar.