ios swift

Override function

I am taking a iOS course online provided by a famous university. I don’t understand why the following code use override and it is legal.

  1. According to the official definition, we use override to override superclass’ methods. Where is the subclass and superclass in the following code?

  2. What’s been override and by what?

public override var description: String {
return "\(url.absoluteString) (aspect ratio = \(aspectRatio))"