user17768: I have been searching for a while for an online (preferably free) resource for opening plans. Up till now my study of openings has consisted of looking at popular lines and some master games. Is there any website or e-book that you are aware of that focuses on the plans behind each opening?

user825: I had this book:

https://www.amazon.com/Encyclopedia-Chess-Wisdom-Eric-Schiller/dp/1580423442

It's not free but it's simple to read for openigs.

user14202: The St. Louis Chess Club has a YouTube page full of video lectures, many of which delve into the opening ideas.  iChess.net also has such videos, for example: "The Ideas Behind the Chess Openings! IM Vlaeri Lilov (Webinar Replay)"
Then there are some wiki pages that actually have decent explanation of some openings.

17768: <p>I have been searching for a while for an online (preferably free) resource for opening plans. Up till now my study of openings has consisted of looking at popular lines and some master games. Is there any website or e-book that you are aware of that focuses on the plans behind each opening?</p>
 825: <p>I had this book:</p>

<blockquote>
  <p><a href="https://rads.stackoverflow.com/amzn/click/1580423442" rel="nofollow noreferrer">https://www.amazon.com/Encyclopedia-Chess-Wisdom-Eric-Schiller/dp/1580423442</a></p>
</blockquote>

<p>It's not free but it's simple to read for openigs.</p>
 14202: <p>The St. Louis Chess Club has a YouTube page full of video lectures, many of which delve into the opening ideas.  iChess.net also has such videos, for example: <a href="https://youtu.be/ZrT5-MxX0R0" rel="nofollow noreferrer">"The Ideas Behind the Chess Openings! IM Vlaeri Lilov (Webinar Replay)"</a></p>

<p>Then there are some wiki pages that actually have decent explanation of some openings.</p>
