It means don't preach the Gospel to someone who is not willing to listen, otherwise they will take what you gave them and tear it apart because they don't want to hear the truth.
Jesus is basically telling His disciples don't try to preach the Gospel to people who are not willing to listen...
Actually it was the Bishops who followed the true teachings of their predecessors the Apostles who defended the Catholic Church's oral and written traditions against the heresies of those who wanted to create their own truths. The Christians were Christians they never called themselves as...