What is the difference between an error, a warning and a notification?

Red, orange and grey: what each colour means and what to tackle first.

Every item on the list has a severity. You can see it by the colour.

Error (red): a visitor or Google will actually run into this. A broken link, a page that doesn’t load. Fix these first.
Warning (orange): it works, but it’s costing you. A title that’s too long, a slow page, a redirect chain.
Notice (grey): good to know, no rush. Often an opportunity to improve something.

Green means there’s nothing to do on that item. Fixed items are crossed out so you can see what’s improved since the last crawl.