Explanatian of (string expected, got nil)?

[..:D&G:..]

Hello, can someone explain to me what the error string expected, got nil means? I see that this accures on many scripts. For example I got an error like bad argument #1 to 'gsub' (string expected, got nil)

[Castillo]

It means that the function expected to receive a string but instead it received "nil".

[..:D&G:..]

Is this a string?

string.gsub 

[Castillo]

That's a function.

[Karuzo]

A string is a sequence of characters.

That means it is e.g. a text, "blabla"

and if it returns nil ,it expected to get a text or something like that but didn't.

Just like solidsnake said

[Gallardo9944]

local errorString = nil -- WRONG, it's not a string. 
local correctString = "nil" -- CORRECT, has " " 
  
local info = string.gsub(correctString,"n","y") -- info will be "yil" - no errors occur 
local infoWrong = string.gsub(errorString,"1","2") -- will return "string expected, got nil" error. "errorString" is not a string