In John Wick: Chapter 2 (2017), can anyone tell me what "The Marker" is and how it works?
Marker is unbreakable oath or contract which must be honoured.
As a form of contract, D'Antonio swore John to a "Marker", an unbreakable promise symbolized by a "blood oath" medallion. D'Antonio presents the medallion to demand services from John, but John refuses, claiming that he is "retired". In retaliation, D'Antonio destroys John's house with a grenade launcher.
Winston, the owner of the Continental hotel in New York City, reminds John that if he rejects the Marker, he will be violating one of the two unbreakable rules of the underworld: no killing on Continental grounds, and Markers must be honored.