Kinetic Energy
Kinetic energy is the energy something has because it is moving.
Think of it this way
A rolling marble on a kitchen counter has kinetic energy. The faster it rolls, the more energy it carries. A slow marble barely bumps a cup. A fast marble can knock the cup right off the counter.
Explaining kinetic energy by grade level
Roll a marble down a ramp and watch it speed up. The faster it moves, the harder it hits things at the bottom. A heavy marble hits harder than a light one at the same speed. That moving force comes from the marble's energy of motion.