"The spine and the heart" mean the essence, or most important part, of something. "Twists and turns" mean many changes of direction, both literally ("There were many twists and turns in the road") and figuratively ("There were many twists and turns in the story").

My interpretation of this is that: "Big words..." --> are words that are not very understandable/not in common use/those they call as flowery words (for example instead of saying, "It was a good day", you say "It was a supercalifragelisticexpialidocious day!", now that's a big word.) "...can make the way dark for those hear what you say and read what you write."--> dark doesn't mean angry here, my dear. It just means it will bring confusion to anyone who intends to listen to you or read those big words. "Make them the spine and the heart of what you speak and write." --> this is like a metaphor, spine means the foundation and heart means the context (or the meaning) of what is spoken or written. "waht's the meaning of twists and turns ?" --> you should spell it as "what" instead of "waht", and twists and turns means the direction of your words. I think it's like saying, "What's your point?" or "Where are you headed to?" or "What are you trying to say?" Well, that's how I think it is.. :) nice question by the way! :)

1. dark = incomprehensible Making the way dark = making stuff incomprehensible 2. spine = backbone of the body that gives its structure heart = centre of body from which all processes emerge heart and spine = (I think the meaning here is obvious) 3. Think of the twists and turns of a long river. Then you have the meaning of this phrase.