It stands for survival instinct and means the reflex that humans have that is meant to help them survive. For example, if you stood on a soft surface, like a couch, and then tried to fall down face forward while remaining completely straight, you would likely not be able to do it. You would probably curl up unconsciously as protection due to si, or you would use your arms to protect yourself, even though there is actually no problem in falling down straight in this case because the surface is soft and you wouldn't get hurt.