Could someone explain to me what metatables are and how to use them?

A metatable is not a table within a table. If it were, this would be valid:

local t = { m = {} }

Is that a metatable? No, it's just a simple dictionary.

A metatable describes what should happen when the table is under certain conditions. I think of them almost like events (although they're not events) because they do A when B happens.

Metamethods are functions in the metatable that fire when something happens. Again, I think of them as similar to events.

Now for an actual example. There's a whole list of metamethods on the wiki that all fire when different things happen. One of them is __call. This metamethod will fire when the table is called like a function. Take a look:

local list = {}
local metatable = {
    __call = function(list, parameter)
        print(parameter)
    end
    }

setmetatable(list, metatable)
list("Hello, world!")

Pretty cool, eh? Normally, if you tried to call a table, you would just get an error.

An important thing to remember is that __call and other metatmethods have two underscores. This is easy to forget.

This is a helpful post on this website on the topic.