Alcohol distillation begins with a fermented product. Distillation is the process of separating the alcohol from the other water or juice of the fermented product. This is done by boiling the wine or beer or mead. Because alcohol boils at a lower temperature than water it steams off first. This steam is caught in a cooling tube where it is turned back into a liquid. And there you have it. Much stronger alcohol that is much easier to move around. The more times this process is repeated, the stronger the alcohol. One distillation usually results in about a 50% strength of alcohol.
The strength of alcohol is measured in a unit known as Proof. Proof is twice the actual amount of alcohol in a mixture. So 40% alcohol is 80 proof. The strength of most of the whiskey and vodka we get in America.
Proof wasn’t always as accurate as it is today. Early American settlers would saturate gunpowder with alcohol and light it. If it flared up, too strong. If it sputtered, too weak. A good strong blue flame was considered proper proof of your alcohol’s content. Usually this is 50% alcohol and 50% of another liquid. So 100 Proof was accepted as quality alcohol.
Also, 100% (not proof) alcohol is not possible because alcohol dilutes itself with water from the air. The US standard for alcohol is 198 Proof.