A Canonical URL allows you to tell search engines that assured similar URLs are principally one and the same. Sometimes you have products or content that is manageable under various URLs, or even on various websites.
Using a canonical URL (an HTML link tag with attribute rel=canonical) these can exist without injuring your Rankings.
A canonical tag is a way of telling search engines that a specific URL represents the unique copy of a page.
Using the canonical tag prevents problems caused by identical or duplicate content appearing on multiple URL s.
Basically, the canonical tag tells search engines which version of a URL you want to appear in Google search results.
The search engine friendly URL that you want the search engines to treat as authoritative. In other words, a canonical URL is the URL that you want visitors to see.