Hi, it is a distillation in vacuum ambient, it reduce the pressure. From Wiki:
''Laboratory-scale vacuum distillation is used when liquids to be distilled have high atmospheric boiling points or chemically change at temperatures near their atmospheric boiling points''
''This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure''
Please don't list the final answer to the people who even didn't bothered to read the wiki article.
This is not the final answer, I tell him the basic principle and a source of information ... sorry Mentor @Borek but the question was so general, he doesn't asked a precise dubt on vacuum distillation he asked what is the vacuum distillation ...