The Head Slot refers to one of the 19 Equipment Slots on the Paperdoll.
Items that can be worn there come in two classes: Hats & Masks, or Helms.
The former can be Blessed using a CBD, and the base Resistances per item total 24. They are, as a general rule, Medable.
Helms, on the other hand, have a typical resist total of 15. They do not count as clothing and hence can only be blessed with an (incredibly rare) IBD. They tend not to be medable.
Despite the apparent advantages hats/masks have, Runic Tools can only be used to enhance helms and so the lighter head wear tends to only be of use in Artifact form. The Hat of the Magi and Kasa of the Raj-In are two examples of popular head gear.