A canonical URL expresses a preferred version of a page where duplicates are possible.
Why this matters
Content-management systems can expose the same article through tracking parameters, archives or alternate paths. A consistent canonical helps clarify which version is preferred, but it is a hint rather than a guarantee of indexing.
Example and interpretation
If both example.com/guide and example.com/guide/?source=email contain the same guide, the canonical should normally point to the stable guide URL. That alone does not correct broken links or intentionally distinct pages incorrectly grouped together.
What to do next
Check canonical tags alongside redirects, internal links and sitemaps. Avoid canonicalizing unrelated pages to the homepage simply to reduce the number of URLs.
Check a website with PageRanker to explore its observable SEO signals. PageRanker provides independent estimates, not Google PageRank.