The more rigid the spring, the larger the spring constant. The displacement is how far back the spring is pulled. The further the spring is pulled, the more energy is stored. As can be seen from the formula, kinetic energy is directly proportional to the square of the speed and to the mass. This means that the more massive an object, the more energy it has, and the higher the speed, the more energy it has.
Kinetic energy is the energy of motion.
It takes more energy to get a massive object moving, and more energy to get an object moving very fast, so we know that they contain more energy.