Mind if I say things in a little simpler way.

Peter is greek for stone, meaning a small rock such as are commonly found. When Christ says "Upon this rock I will build my church," the greek word used means large rock, or a boulder.

He was telling them that Peter was going to be the head of the church, but the church was to be built on a much larger, and stronger foundation, or Christ.

For cross-references see 1 Cor. 3: 11 and 1 Cor. 10: 4