We've encouraged you to look at the Wikipedia page to get a quick read on a source. But you should go the extra mile, and check to see if the page history is substantial and untampered with, and if the talk page raises any issues of which you should be aware.

Talk pages are where Wikipedians express their concerns on various issues around an article. Read the talk page on the Flat Earth article, for example, and you'll learn that there is a debate about whether the page should exist -- it is meant to be historical, about (false) flat earth theories, but it becomes a target for conspiracy theorists (which has resulted in a semi-protected status). Read the talk page for the Atkins Diet and you'll see explanations of why the article only cites summaries of research and not individual studies. The talk page is like the annotations for the main page.
The talk page shouldn't be taken as gospel -- but it often raises issues you might want to take a closer look at.

This video shows you an example of how and when you might want to check talk page.

https://www.youtube.com/watch?v=qIfOio1aCCo