The didrachmon was a silver coin worth two drachmas, the standard amount of the annual temple tax required of every Jewish male over twenty. The half-shekel temple tax, instituted in Exodus 30:13, funded the maintenance of the Temple. Matthew 17 records the remarkable story of Jesus providing this tax in an extraordinary way.
The temple tax miracle is rich with theological irony: the Son of God, the true owner of the Temple, is asked to pay a tax for the Temple's upkeep. Jesus's question — 'From whom do the kings of the earth collect duty and taxes — from their own children or from others?' — establishes His divine sonship and therefore His exemption. Yet He pays anyway 'so that we may not cause offense,' modeling voluntary submission to human institutions even when technically exempt, for the sake of mission.