A redirect loop happens when two or more URLs redirect to each other in a closed circle, A to B, B back to A, so neither the browser nor a crawler ever actually reaches real content. It's usually the result of a configuration mistake: a new redirect added without checking what the destination itself already redirects to, or a conflicting rule from an old migration never removed.
The failure is immediate and visible, unlike a lot of SEO problems that quietly erode performance over time: browsers detect the loop and show an explicit error ("too many redirects") rather than hanging indefinitely, which at least makes it easy to spot once someone actually hits it. The bigger risk is that it can sit undetected for a while if the affected URL isn't one anyone visits often, silently blocking both visitors and crawlers from a page that might otherwise be performing fine. Checking a site's redirect rules for circular references, especially after any migration or bulk redirect update, is the way to catch it before a real visitor or Googlebot does.
