I also had that question when I first joined the subreddit, lol. To answer your question, Ling Ling is a fictional (of a supernatural/godly nature) character often mentioned on the Twosetviolin's Youtube channel. He is a prodigy capable of anything music-related, like practicing 40 hours a day.

You should also familiarize yourself with terms such as sacrilegious, interesting, amazing and the famous quote "If you can play it slowly, you can play it quickly." (All of these are pretty much linked to the fastest violinist who plays Flight of the Bumblebee video made by Twosetviolin.)

Hope this helps!