Lua denotes the string concatenation operator by ".." (two dots). If any of its operands is a number, Lua converts that number to a string.
    print("Hello " .. "World")  --> Hello World
    print(0 .. 1)               --> 01
Remember that strings in Lua are immutable values. The concatenation operator always creates a new string, without any modification to its operands:
    a = "Hello"
    print(a .. " World")   --> Hello World
    print(a)               --> Hello