What Is a Hook Sentence?

In your copy cure masterclass, which again is friggin awesome, I love how you went deep around this idea of hook sentences. Can you tell us what a hook sentence is and why it is so vital to remember when you want to write effective copy? Sure. A hook sentence for me is anything that is like the opening headline, the subject line, the title of a blog post. And it can even be like the first sentence of an article, right? But the reason these things are so important is that if you can't get someone to click on something or to read the first sentence  the likelihood of getting them to read the rest of it is just preposterous.