Swift version: 5.0
Swift uses exclamation marks to signal both force unwrapping of optionals and explicitly unwrapped optionals. The former means "I know this optional variable definitely has a value, so let me use it directly." The latter means "this variable is going to be nil initially then will definitely have a value afterwards, so don't make me keep unwrapping it."
Broadly speaking, using exclamation marks is frowned upon because "trust me it's safe" isn't as good as the compiler absolutely enforcing it. That being said, it's your code: if you know something cannot be nil (usually because if it were nil your program would explode!) then do what works best.
Available from iOS 7.0 – see Hacking with Swift tutorial 1
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.