Override function
<p>I am taking a iOS course online provided by a famous university. I don't understand why the following code use <code>override</code> and it is legal.</p>

<ol>
<li><p>According to the official definition, we use <code>override</code> to override superclass' methods. Where is the subclass and superclass in the following code?</p></li>
<li><p>What's been override and by what?</p></li>
</ol>

<blockquote>
<pre><code>public override var description: String {
    return "\(url.absoluteString) (aspect ratio = \(aspectRatio))"
}
</code></pre>
</blockquote>