The first part of the constant declaration is the let keyword. The let
keyword lets you know that you are working with a constant. The next part
is the constant name s1. You will use the constant name s1 to refer to this
constant in your code from now on.
You also have the type declaration :String. The type declaration tells you
what data type the constant stores. Since you used the type declaration
:String, you know that you can store strings (a sequence of characters) in
the constant s1.