You define a variable in Swift using either the let or the var keyword.
In our playground there is already one variable defined:
var str = "Hello, playground"
A new variable called str with the string value "Hello, playground".
We can create new variables and constant variables of different types:
var myVariable = 123 let myConstantVariable = 123
var keyword defines a variable.
let keyword defines a constant.
Swift is a statically typed language.
Once a variable has a type, it cannot be changed.
We've been letting the compiler work out what it should be.
This means our variables have been implicitly typed.