A ritual is a type of Gear equipped in the Ritual slot, of which there is one. Rituals commonly provide enhancement dice to the Action and Reaction entries of equipped Exploration Expertise and Background. They are used exclusively in exploration scenarios.
Rituals have a level requirement provided. A PC's level must be equal to or greater than the listed requirement in order to equip the ritual. Rituals typically have a Magic entry as well, which is used to add Magic cards to the exploration deck.
Rituals also have an Extra Effect provided, which adds some additional flavor and fun when equipping the card.
For more information, see Player's Guide Chapter 7: Gear and Wealth.