It's a sign of happiness, contentment and submission. We have two cats. We got Two from a restaurant so obviously she was exposed to people. One, on the other hand, was feral and never learned to like people. She has been staying with us for more than two months but we couldn't hold or carry her without struggling to escape. We can only caress her while trying to sleep with Two and when we do that, Two purrs a lot but not One. Her body just stiffens and no sound at all. She curls up as if trying to hide because she's scared.