The Jester is a promo role for Coup which is used as a replacement for the Ambassador.
A player claiming the Jester role may cause Disorder, taking a card from the Court deck and a card from the Target (another player of their choice). The player may secretly change one of their cards for one of the cards received. The player then returns a card to the Court deck and a card to their Target.
The Target may also claim the Jester role to avoid giving (and thus receiving) a role card.
The Target of the Captain may claim the Jester in order to block Stealing.
For more information, visit the BGG listing.