Foreshots are the first vapours to boil off during distillation, usually containing compounds such as acetone, methanol, and aldehyde volatiles. Distillers always discard the foreshots and never allow them to be part of the final product. Depending on the base material used to make the spirit and the apparatus used, foreshots can be 2% to 5% of the overall volume collected.
We always consider foreshots and “heads” as separate parts of the early spirit collection, as for spirits like Gin (or others who are redistilling Neutral Spirit) foreshots are likely to contain the dregs of the previous run left in the tubes. They may also contain some harmful compounds and are always discarded, while the heads are perfectly potable spirit, but simply an undesirable flavour for their recipe.
As the risk of collecting nasty compounds is low when redistilling a pure Neutral Spirit, for gin makers, foreshots are often just 0.1 - 0.2% of the total run, while the heads can be a further 1-3% depending on the recipe. In this case heads are collected separately, added to tails and sometimes used to make other products.
Many distillers who are starting from a wash (i.e. not rectifiers transforming previously distilled Neutral Spirit) such as Scotch Whisky makers and Moonshiners, do not make that distinction and simply separate the distillate as foreshots, hearts and feints.