Energy Level Diagram to basically show you about mechanical system on the energy system.
There is mechanical system on the particle. It is attached or limited spatially that it can only take one specific energy value. Energy levels are those values. It is different with particles which is classical. That difference produce some energy. The term of energy level is usually for the level in electrons which reside in either ions, molecules, or atoms which are related by the nucleus electric field. Here are some example of energy level diagrams.