Distillation is a technique to separate two liquid substances using their different boiling points. A common example is separating water from alcohol. A dilute mixture of alcohol and water is heated in a sealed vessel. As the mixtures approaches 78 deg C  (127 deg F), the alcohol begins to boil and travels into a condenser which is kept cool by a stream of cold water. The alcohol then condenses as concentrated alcohol.
As the mixture continues to heat, its boiling point changes and increasingly more water is boiled off and condenses with the alcohol. Sometimes distillation is done in a series of repeated rounds each time the distillate becomes more and more concentrated.

Essential oils are prepared by distillation.