The stone and the ground are the system in our case. A glass beaker containing hotly reacting chemicals is an example of a system releasing energy to the surroundings as heat. A calorie is defined as the amount of energy required to raise the temperature of 1 gram of water by 1 C and is the preferred unit when talking about heat energy. If one reacts acetylene gas and oxygen in a welders torch, there is a tremendous amount of energy released in the form of heat. An explosion occurs when there is an instantaneous release of energy from a system to the surroundings.
Lets look at the case where our stone is dropped into a pool of water.
The energy of the stone causes a disturbance in the water and waves are generated. These waves appear as a series of crests and troughs. The length of the wave is its wavelength as shown. The wavelength is the distance for one complete positive and negative phase to complete, i.e. a cycle. A stone generates many cycles and the number of cycles generated per second is the frequency.