It stands for Senses Initiated Lucid Dreaming. It's a technique to induce lucid dreams. You're welcome to check out my blog post, although it's a little outdated and not actively maintained: http://cosmiciron.blogspot.com/2013/01/senses-initiated-lucid-dream-ssild_16.html?m=1  

I feel it is not clear and concise in the writing. Also there are additional info I'd like to throw in, such as my hypothesis on why it works and instructions on inducing WILDs etc. I plan to revise it or write something else soon, but first I have to find a better blogging platform. Blogger is just downright horrible.