What is “Fatal error: Unexpectedly found nil while unwrapping an Optional value”?

A swift variable can be any of the following types:

var anVar: Int = 10
var anOptionalVar: Int? = 42
var anNonOptionVar: Int! //Have no valid value, this value is implicitly unwrapped, usually used in dependecy injection
var anUninitializedOptionalVar: Int? // `nil` is the default

An optional variable is a type that represents either a wrapped value or nil, the absence of a value.