Ion temperature is the important metric because it is ions that do the fusion, not electrons.

The sun has immense gravity and mass that produce very high pressures which permit fusion at 'low' temperatures. Maintaining high plasma pressure is difficult on Earth, so higher temperatures are used.

Temperature is fairly well understood as simply the average kinetic energy of the particles of whatever you're measuring. For more nuance, start here: https://en.wikipedia.org/wiki/Temperature#Theoretical_founda...