What Paul is saying is, "I'm going to restrain myself. I will abstain from boasting, but I will tell the truth, that God gave me a vision of the third heaven." He is letting them know that he has superior knowledge and understanding, but he says, "I will refrain from boasting and just give you the truth of it." The application to us is that Paul forebore so that he would not cause offense. He refrained or abstained.