Permanent or temporary
A 301 or 308 redirect tells search engines a page has moved for good, so they move rankings and show the new address. A 302 or 307 says the move is temporary, so Google may keep showing the old address. For moved pages and domain changes, use 301.
Each extra hop slows visitors down and costs crawling time. Point links and redirects straight to the final address.
One version of your site
Your site should answer on exactly one version: usually https:// with or without www. The other versions should redirect to it in one step. This tool checks all four (http and https, with and without www) so you can see if any of them loads separately or takes a long route.
Meta refresh and JavaScript
Some pages redirect with a <meta http-equiv="refresh"> tag or JavaScript instead of the server. Search engines handle these less reliably, so the tool flags them.