Hello, Steve,
Energy is what gives molecules their motion. They have kinetic energy and when they collide with neighboring molecules, some of that energy is transferred. The goal of a calorimeter is to insulate reactions well enough that little energy escapes and they can more precisely measure the energy released or gained by the "system," that portion that contains the reaction. Think of the molecules as bumper cars without brakes. You might slow down a little when some of your kinetic energy is transferred to your sibling's vehicle.
There are also other forms of energy, such as that which is stored in making a bond, and lattice energy, that which is contained in a crystalline structure such as ice. But collision energy is transfer of kinetic energy.
Bob
