coding question

I was hoping someone could educate me on this. Before I change a theme, I run it though sitereportcard.com to see if there are any errors. On a number of themes I try, I get these kinds of errors.

This info is below from the twenty-twelve theme.

Why the errors. How will this affect site ranking and SEO?

Thanks

45 Error: <header> is not recognized!
45 Warning: discarding unexpected <header>
46 Error: <hgroup> is not recognized!
46 Warning: discarding unexpected <hgroup>
49 Warning: discarding unexpected </hgroup>
51 Error: <nav> is not recognized!
51 Warning: discarding unexpected <nav>
55 Warning: discarding unexpected </nav>
57 Warning: discarding unexpected </header>
64 Error: <article> is not recognized!
64 Warning: discarding unexpected <article>
73 Error: <footer> is not recognized!
73 Warning: discarding unexpected <footer>
77 Warning: discarding unexpected </footer>
78 Warning: discarding unexpected </article>
80 Error: <article> is not recognized!