Electrons become excited as they absorb quanta of energy (pieces of energy), usually light, heat, or electricity. In the animation at the right, electricity is flowing past the atom. Occasionally, randomly, the electron is struck by a quantum of electricity. That quantum has a specific energy, which excited the electron to jump up to a specific higher orbital.
Once the electron drops back down, that is called "relaxing", and the electron releases the energy again when it relaxes. Regardless of the type of energy absorbed, the electron always releases light during relaxation. A quantum of light is called a photon.