What the meaning of question mark ‘?’ in swift? [duplicate]

You can use if and let together to work with values that might be
missing. These values are represented as optionals. An optional
value either contains a value or contains nil to indicate that the
value is missing. Write a question mark (?) after the type of a value
to mark the value as optional.

If the optional value is nil, the conditional is false and the code in
braces is skipped. Otherwise, the optional value is unwrapped and
assigned to the constant after let, which makes the unwrapped value
available inside the block of code.

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/pk/jEUH0.l

For Example:

var optionalString: String? = "Hello"
optionalString == nil

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

In this code, the output would be Hello! John Appleseed. And if we set the value of optionalName as nil. The if conditional result would be false and code inside that if would get skipped.