This is correct. To add to it, when you use elements <opening tag>Example words that go on screen</closing tag>, the opening tag gives the browser information about what those words are supposed to do. For example, you’re already learned about <p></p> and <h1></h1>. You see how when you use <p> that it’s just a simple paragraph, but when you use <h1></h1> the words are much bigger? It’s because that opening tag tells the browser what to do with those words. And then the closing tag </p> or </h1> tells the browser that you done using that effect on the words.
So when you used the a tag, you told the browser that you’re making an anchor, but you never told it you were finished making the anchor. And <cat photos/> isn’t a tag at all. The browser doesn’t know what to do with that.