The harmonic mean is a kind of mean (or average) for positive scalar data. The harmonic mean can be expressed as the reciprocal of the arithmetic mean of the reciprocals. See https://en.wikipedia.org/wiki/Harmonic_mean
The harmonic mean of n numbers $x_1 \dots x_n, x_i > 0$ is:
$H = \frac{n}{\sum{\frac{1}{x_i}}}$
Perhaps the most common use of H is when you are averaging rates or averages.