swift !?

Actually what is the "!" in swift? I kNow it is represent NOT meaning,but how come in this case it uses like this? Anyone can explain it to me,I'm new to Swift programming language.

Swift uses!in several distinct ways. The one used in these answers is to force unwrap an Optional value. An Optional type is a regular type,e.g.String,which can also beniland is writtenString?. If you have a value of typeString?and you want to unwrap it to get a value of typeStringthen you use!on the value. In other words,ifx: String?you can writex!to get the plainStringvalue wrapped up by the optional. This only works if you kNow the value is notnil,otherwise you will get a runtime exception. The!is to signify the potential danger.–rokobSep 30 at 5:06