Focus is merely a catch-all term for any item that is required in order to perform or use a certain type of magic. They are commonly required in several magical disciplines in Heroquest:

• Wizardry Spells usually require a Talisman (p. 161 & 164) of some kind.
  • Liturgist blessings, special blessings, and curses often require study of the Church Scriptures (p. 158).
  • Orderly magic spells are listed in the Order's Formulary
  • Adept magic spells are listed in the School's Grimoire (p. 164)
• Animism Charms (p. 134-135) and Fetishes (p. 136) are both Foci.
• Theism magic does not commonly require a specific Foci.