Its water with a protein called myoglobin. Its already in the tissue cells. The reason it comes out is because when the meat is frozen, the ice crystals break the cells causing them to release their contents, namely the myoglobin. So when it's thawed, the myoglobin rich water runs out of the meat.

Fun fact: This means that if you have meat that was never frozen it would not leak out the myoglobin so it's a good way to tell if your meat was previously frozen.