What does the keyword "as" means?

When you assign the result of a previously declared variable to one that is of a different type, you use "as" to format it as the same type in which the variable was first declared. For example, in the Playlist struct, we created a variable called title that was of type String Optional. (var title: String?) When the variable "title" is then assigned to the key "title" in a dictionary, you must say "as String!" because a string type cannot be assigned to a dictionary type. By saying "as String!", you are assigning a string type to another string type instead of a string to a dictionary key.

It's a way of saying something is of a certain Type.

In the Swift Docs it is also described as Downcasting

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TypeCasting.html

Downcasting A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with the type cast operator (as).