Generate a correct rel=canonical tag and a full hreflang set for your language/region variants, with built-in checks for the mistakes that break international SEO.
A canonical tag tells search engines which URL is the "real" one when several URLs could serve the same or near-duplicate content, consolidating ranking signals onto a single page instead of splitting them. Hreflang tags do a related but different job — they map a set of language or region variants of a page to each other so the right version can be shown to the right visitor — and this tool checks for the two mistakes that most often break hreflang: duplicate language codes and relative URLs, since hreflang requires absolute ones.
Search engines will try to pick a canonical URL on their own using signals like internal linking and redirects, which doesn't always match what you'd have chosen — especially for pages reachable through multiple URL parameters, session IDs, or www/non-www variants. Setting it explicitly removes that guesswork.
Search engines treat hreflang as a cluster of mutually-confirming pages — if your English page points to a Spanish page, but that Spanish page doesn't point back to the English one (and to itself), the relationship isn't confirmed from both sides and engines may disregard the annotations entirely rather than guess which direction is correct.
It's a fallback for visitors whose language or region doesn't match any of the specific variants you've listed — typically pointed at your default or original-language version so there's still a defined page to show rather than leaving the choice entirely up to the search engine.