The web client (also known as the website) is the page that makes the magic work. Minecraft doesn’t support audio streaming itself, so the web client is a web page which users must leave open in the background during their visit on your server to hear all sounds.
Users get prompted with a clickable link when joining to open the client, but can also manually re-generate a link using
/media (they all do exactly the same, just whatever fits your server style the best)
It may be a bit annoying to go back and forth between Minecraft and the webclient to get the volume just right, so you everyone can do
/volume to adjust the volume from in game!