A typical website is configured to serve the same content via two addresses: example.com and www.example.com. This redundancy will increase the chances of someone reaching that website directly; but poses a problem for search engines, and you.
Search engines treat a domain and its subdomains as different websites altogether. This means that content available on example.com and www.example.com will be indexed and ranked separately. Thus, a page from your website could be presented multiple times in search results, but not as high as it could possibly get. Moreover, search engines weigh the importance of a page in terms of the number of backlinks to that page. If websites backlink to YourAwesomeArticle.html in multiple ways (specifically by omitting or including the www prefix), this weight will be distributed between these URLs.
