An exciter is distortion applied to one band and mixed back under the original. It creates high-frequency content instead of raising what is already there, which is why it can bring detail back to a recording that has nothing left above 10 kHz to boost.
Frequency
Frequency sets the corner of the high-pass that feeds the harmonic generator, from 1 to 12 kHz.
Everything under it is passed straight through untouched. Only the band above it is distorted, and only the result of that is added, so the body of the sound never enters the process.
Higher settings are safer. At 2 kHz the generator is being fed vowels and midrange, and the added harmonics land in the region the ear is most sensitive to.
Amount
Amount scales how much of the generated signal is mixed in, from 0 to 100%. At 0 the file is untouched, which is what the step does when it sits in a chain without being engaged.
Because the harmonics are created from the input, the effect follows the material: loud passages get more, quiet ones get less. That is the difference people hear against a static EQ boost.
Odd and even character
Odd order distortion is symmetrical clipping. It produces the third, fifth, and seventh harmonics, which sit a musical fifth and beyond above the fundamental and read as edge, bite, or presence.
Even order distortion squares the signal. It produces the second and fourth harmonics, an octave and two octaves up, which read as air and are harder to notice as distortion. The DC that squaring generates is filtered out at around 20 Hz before anything is added back.
Where it hurts
Cymbals, sibilance, and digital noise are already high-frequency content. Feeding them into a harmonic generator makes them harsher rather than more detailed.
Material that has been through a low-bitrate codec is also a poor candidate: the artefacts near the top of the band get excited along with everything else.
Example: a flat vocal recording
Frequency 4 kHz, amount 30%, odd character.
Consonants gain definition and the take moves forward in the mix without a shelf that also raises the room and the preamp noise.