Different liquids boil at different temperatures
Water boils at 100 °C. Alcohol boils at 78 °C. That small difference is a tool: if you gently heat a mix of the two, the alcohol turns to vapor more eagerly than the water.
Catch that vapor, cool it back to liquid, and you've got something richer in alcohol than you started with. Do it over and over, stacked up in a tall tower, and you can get very pure products. That tower is a distillation column — the most common separation in the world.
The substance that boils more easily concentrates in the vapor.