Hey, @zhangsiya24
I assume you don’t understand what <!-- TODO: Add link to cat photos. --> means.
That is a comment. The browser ignores comments, but they are useful for humans. You can write notes, explanations, todos, etc. for your future self or for other people. In other languages comments look like this //elaborate description or this /* many words */ or this #abababababanana. In html they start with <!-- and end with -->.