Redirect chain
Also known as: redirect hops, 301 chain
A sequence of two or more redirects between a requested URL and the page that finally answers.
Chains accumulate: http to https, then non-www to www, then an old slug to a new one, then a trailing-slash normalisation. Each hop is a full round trip, paid by every visitor and every crawler on every visit.
Collapsing them means pointing the first redirect directly at the final destination rather than at the next redirect.
One hop is normal and costs almost nothing. The thermometer allows one for exactly that reason. Four is a sign that nobody has looked at the redirect rules since the last three site changes, and that is usually true of more than the redirects.