An Exciter (or enhancer) adds a particular type of sparkle to sounds. It appears to create brightness from sounds that were ‘flat’. Traditional exciters work by picking off the higher frequency part of the sound, compressing and dynamically altering it, filtering off the original sound and remixing the resultant harmonics back with the signal. Some units intentionally distort the split off high frequencies to add even more upper harmonics. It adds high frequency sparkle, making singing voices or instruments sound more present and exciting without adding the hiss and phase problems you get from simply boosting high frequencies with an equalizer. Used properly, it can create beautiful sounds. Overused it can be horrible. When it comes to applying an exciter, it’s best to save it for final mixdown as an insert effect since it’s purpose is generally to bring something forward or sit better in an overall mix. If you track with it, once it’s printed, there’s no way to fix it if it sounds bad.