Once more Jesus spoke to them in parables, saying: ‘The kingdom of heaven may be compared to a king who gave a wedding banquet for his son. He sent his slaves to call those who had been invited to the wedding banquet, but they would not come. Again he sent other slaves, saying, “Tell those who have been invited: Look, I have prepared my dinner, my oxen and my fat calves have been slaughtered, and everything is ready; come to the wedding banquet.” But they made light of it and went away, one to his farm, another to his business, while the rest seized his slaves, maltreated them, and killed them. The king was enraged. He sent his troops, destroyed those murderers, and burned their city. Then he said to his slaves, “The wedding is ready, but those invited were not worthy. Go therefore into the main streets, and invite everyone you find to the wedding banquet.” Those slaves went out into the streets and gathered all whom they found, both good and bad; so the wedding hall was filled with guests.
‘But when the king came in to see the guests, he noticed a man there who was not wearing a wedding robe, and he said to him, “Friend, how did you get in here without a wedding robe?” And he was speechless.Then the king said to the attendants, “Bind him hand and foot, and throw him into the outer darkness, where there will be weeping and gnashing of teeth.” For many are called, but few are chosen.’
I'm including the preceding story since the verses you're asking about are the conclusion to the parable.
I think the key to understanding this parable is to remember that Jesus tells us that 'the kingdom of heaven is within.' So everyone—the king, the servants, the invited guests—are aspects of our own ego consciousness. The Christ of our being is always inviting us to a 'banquet'—a life of abundance and joy. Ego consciousness finds as many 'practical' distractions as it can to make us forget the original spiritual commitment that brought us into this human experience. Our more advanced thoughts are particularly adept at finding these excuses. But the kingdom is for us in our entirety, so that part of us that is having little success at being human may lead us to the kingdom more quickly. But we can't stay in the kingdom experience if our consciousness remains in lack and duality. So many—indeed, all!—are called, but it takes a deep spiritual commitment to stay.