In US politics, a PAC is a political action committee: a group set up to collect money and spend it to help or block candidates and causes. Companies, unions, and people who care about one issue often sit behind a PAC. Under US rules it is a legal way to pool money for elections, so you hear the letters constantly in American campaign news. Taiwan does not use this exact kind of group, so the English letters usually stay as they are. Though actually, not every group that gives political money is a PAC — the name is a specific US organisation. A later, looser kind called a Super PAC can raise still more; that is related, but it is not the same label.