This is a reference to something that happened earlier in Acts, in the previous chapter. In Acts 25 Paul is on trial before Festus, who says to him, ‘“Do you wish to go up to Jerusalem and there be tried on these charges before me?” But Paul said, “I am standing before Caesar’s tribunal, where I ought to be tried. To the Jews I have done no wrong, as you yourself know very well. If then I am a wrongdoer and have committed anything for which I deserve to die, I do not seek to escape death. But if there is nothing to their charges against me, no one can give me up to them. I appeal to Caesar.” Then Festus, when he had conferred with his council, answered, “To Caesar you have appealed; to Caesar you shall go”’ (Acts 25:9-12).
In the next chapter of Acts (which is still before Paul has gone to Caesar’s court in Rome) Paul is on trial again. The people Paul is on trial before say Paul is innocent (Acts 26:31-32), so he would have gone free, were it not for the fact that he had already appealed to go and stand before Caesar’s court.