
Harmonicas have variations in how they are tuned. Temperaments refer to how close they are tuned to "perfect". Equal Temperament is like a piano. However, many harmonicas are tuned to compromise - which tried to have single notes a few cents sharp or flat to make better sounding chords. There are other tunings like 7 limit or 19 limit just intonation that prioritize better sounding chords than single notes.
Honestly, most people have a hard time hearing the difference between equal temperament and compromise tunings.