String Manipulations? "String." Question

I'm a bit confused with String.(WhateverHere). What i'm confused of is the String part. I'm asking about:

string.len() --The string part
string.lower() -- The string part, Function doesn't matter, I'm showing it doesn't matter by showing two different functions

I've noticed I can do something like this:

plr.Chatted:connect(function(msg)
	if string.lower(string.sub(msg,1,5)) == "load " then
	end
end)

and the script knows that the string is the message. Yet if I were to do something like this in a script:

for _,Map in pairs(MapTable) do
	if string.match(string.lower(string.sub(msg,6))) == string.lower(Map.Name) then
	end
end

I'll be thrown an error because the script doesn't know the string it's looking for?

• Can I be given an explanation on what it's doing?

• Can't string be switched out with, well, a string?