The short answer is that chemically, chocolate is composed of several different little crystals (six to be exact) but the desirable ones are called beta crystals. The development and formation of these beta crystals are what makes well-tempered chocolate. If you imagine a cup of chicken stock, it may seem uniform, but if left to sit, the fat will rise to the surface.
There are different methods for tempering chocolate. Some are a bit complicated, and some are really messy, especially for home cooks.
Check out this simple 3-step method: https://www.davidlebovitz.com/tempering-choco/
Temper chocolate perfectly every time with a chocolate tempering machine! Compare machines here: https://www.chocolatetemperingmachines.com/compare/chocolate-tempering-machines