It's definitely a tricky topic with a bunch of things with very similar names. Hopefully, this can help you out.
There are three different things
1) The HTML Head Element which is <head> - Is is a tag and the <body> used to break up the information in an html file into two sections. The <body> tag hold all the content of the page and the <head> holds all the extra information often referred to at META data. You can go to https://www.w3schools.com/html/html_head.asp for more information.
2) The Header tag which is <header> - You don't need this tag for CS Discoveries. For more information on it, you can go to https://www.w3schools.com/tags/tag_header.asp .
3) HTML Headings which are tags <h1> , <h2> ... <h6>. These show on your page because they are the section headings for your content on your page. We sometimes refer to them in the curriculum as headers. These will be one of the most common tags that students will use throughout the curriculum. For the purposes of what students are working on early in the curriculum these are the most important tags of the 3 listed here for them to understand. For more on these you can go to - https://www.w3schools.com/html/html_headings.asp
Hope that helps!