`nil`

In Swift, nil means the absence of a value. Consequently, nil can only be assigned to optional values in Swift. Optionals are a key concept of Swift, I recommend reading Apple Docs: Swift - The Basics

One example would be an integer: we want to know whether a value has been assigned to it and act accordingly. Back in the days you would initialize this variable with 0 or -1 and check against these values to see if the variable has already been changed or not. But what if 0 or -1 were valid values as well? Nil is a much better approach, as a nil value definitely means the absence of a value, so we don't have to worry about initializing optionalInt with some arbitrary number.

// Error, nil cannot be assigned to a non-optional value
let optionalInt: Int = nil 

// Works, as we declare optionalInt as optional with ?
let optionalInt: Int? = nil

// As optionalInt is optional, we have to unwrap this value
// "optionalInt is nil"
if let unwrappedInt = optionalInt {
    print("optionalInt is not nil")
} else {
    print("optionalInt is nil")
}

// Now we set the value to an actual integer
optionalInt = 1

// "optionalInt is not nil"
if let unwrappedInt = optionalInt {
    print("optionalInt is not nil")
    // Now we can use unwrappedInt here, it is now
    // unwrapped and therefore not optional anymore
} else {
    print("optionalInt is nil")
}

This is just a very basic example, there lies great potential in how and when to use nil, so make yourself familiar with the optionals concept :)

Hope that helps :)