What does nil mean in Swift?

Question

Swift

Sorry for the possibly dumbest question, but I'm wondering what nil means by itself and in this context:
var SampleT = "123"
var Sample = SampleT.toInt()
if Sample != nil {
println("100 + 23 = \(Sample )")
}
var SampleText: String? = "Hello"
SampleText == nil

Answer 1

nil analogue of NULL
in this case, as an option, an empty or uninitialized string

Answer 2

nil is a constant with a "missing" value - or a nonexistent value. Many functions return this value on failure and error.