What the status codes mean
2xx means the page loaded: 200 is what every page you want in Google should return. 3xx is a redirect: 301 and 308 are permanent, 302 and 307 temporary. 4xx means the page isn't there or isn't allowed: 404 not found, 410 gone for good, 403 forbidden. 5xx is a server error, which search engines treat as a temporary problem and retry, but too many slow down crawling.
This tool follows each address through up to 3 redirects and shows the whole path, so you see both the first answer and where it ends up.
When to use it
After a site move, to check that old addresses redirect to the right new pages. After deleting pages, to check they return 404 or 410. And to check a list of pages from Search Console reports quickly.