What you need to know
Query parameters, trailing slash choices, and alternate routes can create multiple URLs that show the same content. A canonical link is a hint that identifies the URL you want search engines to treat as primary.
How to apply it
Choose one stable public URL for each page, point the canonical tag to it, and use that same URL in internal links and the sitemap. Confirm that the chosen URL returns the intended page.
Common mistake to avoid
Do not canonicalize every page to the homepage. Canonical URLs should represent equivalent pages, not unrelated content.