Neutral spirit is a highly purified, nearly odorless and flavorless distilled alcohol, typically 95% ABV or higher, produced by distilling fermented grains or other starches to a very high proof.
Because it carries almost none of its own flavor, neutral spirit is the base for vodka, for gin (which gains its juniper-forward character only after being redistilled with botanicals), and for many liqueurs, bitters, and infusions. In the kitchen it's used to flambé desserts, to extract flavor from herbs, spices, and botanicals for tinctures and infused oils, and, once fermented further, to make certain vinegars.
The key distinction to keep in mind: neutral spirit itself has no character, so any flavor in a finished product, whether vodka, gin, or a liqueur, comes from later processing or infusion, not from the base spirit.