Some of these probably warrant investigation. A lot of it is stuff that you can do yourself, though.
Internal pages
404 page
4XX page
^^^ Seems to indicate there are dead links somewhere on your website, by which I mean you have links to pages that do not exist.
Links
Orphan page (has no incoming internal links)
^^^ Seems to indicate that there is at least one page (maybe more than one) that are seemingly inaccessible from your website. For example, maybe have a Privacy page, but do not provide any links to that page. FWIW, I usually put the Privacy link in my page footer, not in the menu. But it could be any old page, or an old album or something that you should consider cleaning up, pages from a previous version of the site, etc.
Content
Meta description too short
Meta description tag missing or empty
You should fill in the Meta Description for all of your albums and pages in Backlight. Edit each album or page to do so, and you should see the box above the main content input.
They probably expect the CSS to be minified; we don’t do that. I wouldn’t worry about it. I mean, it’s sort of a bullshit error. CSS is as long as it needs to be for any given website. There’s always opportunities for optimization, but that’s neither here nor there. “You’re CSS is too long,” is a stupid error, given that whatever linter they’re using is incapable of considering context; it’s either looking at file length (number of lines) or file size, and whatever value the linter is configured to use as a threshold is entirely arbitrary.