I believe the analogy preceding this verse reveals the answer to that question. The host sent messengers to Israel, and they rejected the invitation, so he sent the messengers to the rest of the world (everyone). Many responded to the invitation, but some didn't come clothed in the righteousness of Christ obtained through faith (wedding clothes).
Thus, the "many called" refers to those who hear the invitation of the gospel appeal. The "few chosen" refers to those dressed in the proper wedding garments, the righteousness of Christ obtained through faith alone.