How can a liquid go past its freezing point and still be a liquid?

I dont understand the concept of super liquid water. I saw on flight 447 (a plane that crashed due to super liquid water) that they went into a huge storm 35000 feet above the ground. It sais that the rain was past the freezing point and once it touched the planes pitot tubes it froze them, therefore the tubes couldnt function and they couldnt tell the pilots how fast the plane was going therefore they ended up stalling. Can someone explain in an easy-to-undersatnd way how super liquid water works?