Paul indicated in 1 Corinthians 9:12 that he did not use the right he had to be compensated for his ministry as an apostle so that the gospel would not be hindered. He chose rather to endure all things, including suffering need that the gospel might advance (cf. Philippians 4:10-13).
In verses 19-23, he indicated that for the sake of the gospel, he was willing to become all things to all men in order that some might be saved. Paul wrote, “and to the Jews I became as a Jew, that I might win Jews; to those who are under the law, as under the law, that I might win those who are under the law; 21 to those who are without law, as without law (not being without law toward God, but under law toward Christ), that I might win those who are without law; 22 to the weak I became as weak, that I might win the weak. I have become all things to all men, that I might by all means save some (1 Corinthians 9:20-22).
Paul knew he must “flex” in order to be more effective and fruitful in preaching the gospel. By being flexible he meant that though he was free from all men, he made himself a servant to all. He had liberty as a Christian, but he was willing to limit that liberty in order to reach more people with the gospel of Christ.