The following code creates a constant and
stores the string "Hello World" in a constant named s1.
let s1:String = "Hello World"
The first part of the constant declaration is the let keyword.
The let keyword is used to define a constant.
The next part is the constant name s1.
You also have the type declaration :String.
Variables use the var keyword instead of the let keyword
to declare.
Variable values can change.
var s2:String = "Hello World" s2 = "Hola Mundo"