This video explains a lot about sections and valid code. He talks about how using tags like <section> and <article> tags can make the code more human-readable and help search engines like Google display information about your site better. Also, he talks about invalid and valid code. He says that the <h1> can not be in another h1 tag. This would be invalid code:
<h1> <h1>Invalid code</h1> </h1>
It would display properly, but it is best not to do this because it can confuse web browsers sometimes. And last of all, he talks about h1 through h6 tags, footers, and headers.