Jurgis has to control his temper or else he will get in a lot of trouble.
He is angry because of all the people who are cheating the family out of money. The guests have come to the wedding feast and are happy to eat and drink what is provided. But many of them are cheating and are unwilling to help pay for the meal the way custom says they are supposed to. The bar owner is cheating the family by giving worse quality than they paid for, but nothing can be done because he has political connections.
Jurgis has to hold his temper down because getting mad at them will only get him thrown in jail or get him in trouble with the powerful people in the community.