Distillation is the process of separating ethyl alcohol from the contaminates that boil off before the Etoh (foreshots/heads) and those that boil off after the Etoh (tails). Foreshots are poisonous...the Etoh is then collected (the hearts) until the tails start showing up. The tails can add flavor and are generally added in small amounts...they are, however, responsible in part for hangovers. Pure Vodka has no heads or tails...hence pure and less likely to make you feel badly.
for additional info, here is a brief descriptioin:
https://www.whiskeystillpro.com/blogs/news/31769601-how-to-cut-your-alcohol-distilling-run