Internal energy is the energy related to the random motion of molecules or the thermal energy of a contained system. It is microscopic energy on the atomic and molecular scale.
Scientists in thermodynamics generally use "E" to represent internal energy, though many continue to use "U."
The internal energy of a system cannot be determined until a change occurs within that system. For example, a glass of cold water has a constant internal energy, but upon heating, a change in internal energy occurs, allowing a scientist to determine the original internal energy value. Freezing water is an example of a decrease in internal energy because the molecules slow down as water changes to ice.
The value dE represents the change of internal energy within the system caused by an input or output of energy. Commonly, heat (h) or work (w) are the main causes for a change in internal energy. Adding or subtracting matter can also cause changes.
Hyper Physics further explains that a glass of water seems to have no energy sitting on the table, but at the molecular level, there is an incredible amount of kinetic energy and potential energy between the molecules due to attractive forces and movement.