Jamieson, Fausset and Brown on Matthew 17:24

24. And when they were come to Capernaum, they that received tribute money--the double drachma; a sum equal to two Attic drachmas, and corresponding to the Jewish "half-shekel," payable, towards the maintenance of the temple and its services, by every male Jew of twenty years old and upward. For the origin of this annual tax, see Ex 30:13, 14; 2Ch 24:6, 9. Thus, it will be observed, it was not a civil, but an ecclesiastical tax. The tax mentioned in Mt 17:25 was a civil one. The whole teaching of this very remarkable scene depends upon this distinction.

came to Peter--at whose house Jesus probably resided while at Capernaum. This explains several things in the narrative.

and said, Doth not your master pay tribute?--The question seems to imply that the payment of this tax was voluntary, but expected; or what, in modern phrase, would be called a "voluntary assessment."