做主 (zuò zhǔ) means 'to make the decision' or 'to be in charge of deciding', emphasizing that you have the authority or final say over something. It's often used as a separable verb (做...主), e.g., 做不了主 'cannot decide'. Contrast with 决定 (juédìng), which is a general 'to decide' without the authority nuance, and 主宰 (zhǔzǎi), which means 'to dominate/rule' with a much stronger, controlling sense.