"Not to worry...He won't leave the circle. You found life in the desert. the omen that I needed." This is what the Alchemist tells Santiago as he draws a circle around the snake. In my perspective, this passage isn't about the snake at all, it's about the crystal merchant. The passage means that if you're stuck in a circle (in the merchant's case it's his job of selling crystals), it's difficult getting out.
"Usually I learn more from my sheep than from books." This is something that Santiago tells the merchant's daughter when she figures out he's able to read. I think what the author is trying to make us realize is that knowledge doesn't only come from books. Of course they're helpful, but you should also look around you for inspiration and you could learn a thing or two.
"But I'd like to see the castles in the towns where they live." This is something Santiago tells his father after being told that all the explorers dream of living where Santiago was at. The meaning of this passage is that you should go out and live your own dreams and do what makes you happy, not what makes others happy.
"There is only one thing that makes a dream impossible to achieve: The fear of failure." This quote from the book is pretty straightforward and it's meaning is inspiring. The quote means if there is a dream that you strive to achieve, the only thing that stops you from achieving it is your fear or not being able to accomplish it. If you don't try, your chances of achieving it are absolutely zero.
"When you want something, all the universe conspires to help you achieve it" This quote means that if you're hoping to accomplish something in life, the universe will help you achieve your goal. The only thing that's keeping you from going out and achieving your goal is yourself.