An energy level is the measurement of discrete energy a subatomic particle, such as an electron, can absorb. When light or other energy strikes an atom, it can transfer some of that energy to its particles, raising their energy level.
Under normal circumstances, all of an atom's electrons exist in the ground state, occupying the lowest-energy orbitals available. When energy strikes an atom, there is a chance some of that energy transfers to the electrons, effectively moving them into higher-energy orbitals. This is an unstable state, so the atoms typically give up this extra energy through vibration or emitting energy themselves.
A fluorescent light relies on this principle. Excited electrons strike atoms inside the gas tube, transferring energy. The atoms expel this energy as photons, which strike the phosphorescent coating of the tube and produce visible light.