Metatables

Metatables are the underlying implementation of objects in Lua, they define custom behaviors such as addition between 2 objects, subtraction, accessing the value, etc.
Each of those are called metamethods, and they all begin with __.
All metamethods (Lua 5.1)
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       __eq :     Equality      :  a == b
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       __le :     Less than     :  a < b
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       __ge :   Greater than    :  a > b
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      __add :     Addition      :  a + b
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      __sub :    Subtraction    :  a - b
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      __mul :   Multiplication  :  a * b
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      __div :     Division      :  a / b
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      __pow :   Exponentation   :  a ^ b
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      __unm :     Negation      :  -a
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      __mod :      Modulus      :  a % b
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   __concat :   Concatenation   :  a .. b
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__metatable :     Metatable     :  getmetatable(a)
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     __call :       Call        :  table()
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    __index :     Indexing      :  table.x | table['x']
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 __newindex :     New Index     :  table.x = 'abc'
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     __mode :  Weak References  :  mt.__mode = 'k'
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 __tostring : String Conversion :  tostring(a)
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By default whenever you have a string, it has a metatable attached to it as you can see here:
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local s = "abc"
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​
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print(getmetatable(s)) -- {__index = table: 0x56458af94c30}
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​
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for k, v in pairs(getmetatable(s).__index) do
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	print(k, v)
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end
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When attempting to index the string, it instead returns the string library table. Which is why if you run this code, you'll see the __index of the metatable print this out:
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dump	function: 0x557e9eddcba0
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byte	function: 0x557e9eddcd70
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reverse	function: 0x557e9eddba30
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lower	function: 0x557e9eddbcd0
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gsub	function: 0x557e9eddd810
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upper	function: 0x557e9eddb980
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sub	    function: 0x557e9eddcc50
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match	function: 0x557e9eddd7f0
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gmatch	function: 0x557e9eddc000
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rep	    function: 0x557e9eddbae0
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len	    function: 0x557e9eddbc70
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char	function: 0x557e9eddc060
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format	function: 0x557e9eddc1c0
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find	function: 0x557e9eddd800
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You can directly access these by indexing the string directly, instead of going through the string library:
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local s = "abc"
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print(s.len(s)) -- 3
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print(s:len()) -- 3
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The second print uses the : syntactic sugar, both are equivalent, but the second is preferred for shorter code. The left hand of the : will be inserted as the first argument to string.len, and will result in the equivalent code: string.len(s).
You can create your own tables and define metatables for them to do as you'd like, like so:
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local t = {1, 2, 3}
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​
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print(tostring(t)) -- table: 0x55b410c31620
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​
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setmetatable(t, {
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    __tostring = function(self)
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        return table.concat(self, ', ')
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    end
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})
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​
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print(tostring(t)) -- __tostring invoked, output is 1, 2, 3
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Here we define a custom metamethod for the tostring function for the table, which instead calls table.concat to concatenate all table values in sequence with a separator of , inbetween each value.

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