What is the canonical issue?

For example if you have the website http://www.example.com then the index page can be accessed from all of the following URLs:

What is the canonical issue?

http://www.example.com
http://www.example.com/index.html
http://example.com
http://example.com/index.html

Source page - https://www.koozai.com/blog/search-marketing/how-to-resolve-the-canonical-issue-on-your-website-927/