Heads are part of the spirit that flows at beginning of a distillation run. Typically, they contain a high percentage of low boiling alcohols and other compounds such as aldehydes and ethyl acetate.
For many spirits, heads are recognizable by their distinct astringent aroma (kind of solvent-like and not very pleasant). Some distillers will discard them, others recycle them for other purposes such as making hand sanitizer or antifreeze.
See Foreshots