What are saturators/harmonic exciters?
Saturators/harmonic exciters are effect plugins that are sometimes used when mastering. They’re not mandatory, only sometimes used. When applied, they add intended distortion to your audio. Subtle distortion can sometimes add some character to your sounds.
They usually operate like multi-compressors, in that you first select frequency bands, and then apply distortion. Generally, you want less distortion applied to your lower frequencies than your higher frequencies.