The excitation wavelength describes the light shone on a sample to transfer energy to ("to excite") a light-reactive chemical group in any compound. Its unit is usually given in nanometers (nm).
An example is the green fluorescent protein (GFP), which can be excited with light of 488 nm, to induce a fluorescence at 509 nm.
Excitation wavelength is based upon how much energy a specific group can absorb and reemit. Energy, however, can only be transferred in discrete units called quanta. Based upon the electron configuration of each chemical group, its electrons will be able to absorb a certain amount of quanta and move up in energy level. Upon returning to their original states, the electrons release more of these packets of energy, though at a different level. The release of this energy is the level of fluorescence achieved.