What a redirect chain costs
Every redirect is a full round trip before the real page starts loading - on a mobile connection, often 100-300 ms each. Chains build up over years: http to https, then www to non-www, then an old slug to a new one. Googlebot follows up to ten hops, but each one wastes crawl time, and visitors feel every one of them.
301, 302, 307 and 308
301 and 308 are permanent: search engines treat the new address as the one to show and index. 302 and 307 are temporary: the old address is kept in mind. Using a temporary redirect for a permanent move - a common default in some frameworks - sends a weaker signal. 307 and 308 also keep the request method, which matters for forms and APIs.
The four versions of your address
http://example.com, http://www.example.com, https://example.com and https://www.example.com are four different URLs to a search engine. All four should end on one, ideally in a single hop. The checker traces all of them, so you can see whether any take two steps or end somewhere else.
This follows HTTP redirects - the ones the server sends. Redirects done in JavaScript or with a meta refresh happen in the browser after the page loads; check those pages with the SEO checker. For response headers and the HTTPS certificate, use the security headers checker.
