Hi Darken5,
Here are two video links that I personally find useful.
https://www.youtube.com/watch?v=CYxMfVy5W00
This one seems to explain the brief concept of metatable in Lua..
https://www.youtube.com/watch?v=22QTG81kJaQ
After I posted a previous reply, I've done some further research, because I was unsure of metamethod and this video explains on how you could implement them.
Now I think I got why SDPhantom had used metatable in his example
https://www.lua.org/pil/13.html
This is just a link to official lua documentation page of metatable section.
Hope this helps!