When I was a piskie I asked one of our ministers (I don't call them priests anymore) what this verse meant and he said nobody really knows what it means. Now that I'm Catholic I find that it is the Rock of Scripture on which our whole church is built. Lovely.
I firmly believe everyone knows what this means. They just don't want to accept it. I find it ironic that even though people will argue endlessly that when the Bible says "God created the world in 7 days" it means literally seven 24-hour days, they will not take literally many of Jesus' words, such as this one and His statement that we must eat His Body and Blood. Even though God "really created the world in seven 24-hour days," Jesus "didn't really mean that" when He said difficult things. It is selective acceptance of the Truth of Scripture.