Within the study of computer science, canonicalization refers to the process of converting data with more than one representation into a standardized form. Within SEO, it’s about sending the right signals to search engines about multiple versions of the same page. When multiple pages have the same content but different URLs, the ranking power of those pages get split up, unfortunately.
Example: These are four versions of one homepage: www.homepage.com; homepage.com/; www.homepage.com/index.html; and homepage.com/home.asp. Which one is the “right” one?
SEO experts can use a 301 redirect or a rel=“canonical” attribute to point the user — and the ranking power — to one location. It also helps when a website consistently links to a single version of a page.