A redirect chain occurs when there is more than one redirect between the initial URL and the destination URL. When a URL is redirected, it should have a single 301 redirect in place. Most redirect chains happen inadvertently for one of two reasons:
Reason #1: Redirect chains are built over time. URL A is redirected to URL B, then a year later URL B is redirected to URL C. Reason #2: A combination of HTTPS vs. HTTP, www vs. non-www and trailing slash vs. non-trailing slash redirects results in site-wide redirect issues. Commonly done during website migrations or relaunches where redirects are built in series instead of in parallel.