During distillation, fusel alcohols and fatty acids from yeast are concentrated in the "tails" at the end of the distillation run. They have an oily consistency, which is noticeable to the distiller, hence the other name fusel oil. In a reflux still they can be separated out almost completely if desired. If given time, low wines with go through a phase separation and the tails will collect at the top of the container. If racked from the bottom the tails portion can be reduced for the spirit run.
Tails can also be saved up until there is enough of them to charge up the spirit still, and run as an all tails run. All tails liquor, especially from grain mashes, is usually strong on flavor when run through a pot still. All tail liquor can stand up to heavy oaking without being overpowered by the wood flavors. These types of liquors can be referred to "Reserve" runs or "Queens Share".