Canonical URL is the URL chosen by search engines out of several seemingly similar URLs. Generally, it is related with homepages of a website. Most people would consider the following URLs as one and the same.
www.seodictionary.net
seodictionary.net/
www.seodictionary.net/index.html
seodictionary.net/home.asp
However, technically speaking they are different URLs A web server could return completely different content for all the urls above. To help search engines pick the best URL out of many, webmasters need to follow consistent link structures in the websites.
Google picks up the url that a webmaster wants and use that url consistently throughout their site. For example, don’t make half of your links go to http://seodictionary.net/ and the other half go to http://www.seodictionary.net/ . Instead, a webmaster should pick the url they prefer and always use that format for the internal links of their website.