Why can I use a constant initialization as if-condition?

While reading the official Apple Guide I found this

var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello \(name)"
}

There is a constant declaration and assignment where I — as a beginner — expected an expression returning a boolean. But the condition of this if statement seems to get true as the value, because the code inside the parentheses is being executed.

Does the initialization of a constant return a boolean value or something that a if statement can use as a condition?