Markers are a blood oath between two individuals.
Also, markers are formally witnessed or recognized; It is not just any blood oath.
A marker is small round metal object indicating the debt of a blood oath between two individuals. Opening in the middle to reveal a divided surface, the debtor presses a bloody thumbprint on one side to commit an oath is owed, while the debtee likewise presses their bloody thumb to the other side to indicate when an oath has been fulfilled.
Records of blood oaths are registered and tracked by The Continental, under the supervision of Winston. He tracks the issuance and redemption of blood oaths in his own leather-bound record book.
When John Wick wanted to get out of the business to marry Helen, he was first assigned an “impossible task.” In order to succeed, he gives a blood oath to Santino D’Antonio, the influential son of (later brother to) one of the 12 members of The High Table, in return for his aid. (At the time the marker was given, D'Antonio senior, the art collector, was the representative for the Camorra at the High Table.) After John comes back from retirement to seek vengeance on Viggo Tarasov, Santino calls in the oath. Desiring to take his sister’s place at the table, D’Antonio has John Wick assassinate her to satisfy his commitment. Afterwards, Winston compels D'Antonio to acknowledge the Marker's fulfillment.
After John has a $14 million bounty placed on him and is made excommunicado from the Continental Hotel, Winston gives John a Marker between John and Winston in case he has need of Winston's help in the future.