Taking a look at the Gnome Wiki, and knowing that it is a popular protocol for sharing music and video Streams, the Share option from gnome uses
UPnP. Quoting Gnome Wiki:
Share your music, photos and videos
You can browse, search and play the media on your computer using a
UPnP or DLNA enabled device such as a phone, TV or game console.
Configure Media Sharing to allow these devices to access the folders
containing your music, photos and videos.
Having an equipment that is "DLNA Ready" like a smarttv or even a media player that can listen to dlna, and be placed on the same network is enough to receive the sharing stream.
Depending on the manufacturer, the product may use a branded version
of DLNA such as SmartShare (LG), SimplyShare (Philips), or AllShare
(Samsung), but rest assured it’s all the same technology and it will
From getting started with DLNA.
Keeping the answer short: It will map a folder of your computer that has media in, and create some sort of "sharing" through
dlna. You will need to have at the other side, a client(hardware or software) that can handle dlna streams. The same dlna server setup can be created with
minidlna. Some servers can "send" media stream to a waiting client, and not only provide a share.