What does it mean that the rejected stone will become the head of the corner?
It means Christ, whom the builders rejected, is the chief cornerstone who holds everything together. Though counted as nothing, He will yet one day fulfill that role as the head of the corner, and salvation is found in no other name.
Les explains that Peter is applying the stone reference to Christ. The builders should have recognized Him as the chief cornerstone, the one who would hold the whole thing together, but instead they pitched Him aside and did not know what to do with Him. The stone set at naught, or counted for nothing by the builders, is to become the head of the corner; He will yet one day fulfill that role. Les also points out that Jerusalem itself is a stone of stumbling, and the nations of the world stub their toe on Israel and Jerusalem right up until the last day. The passage then adds that there is salvation in no other name. Even devout, sincere Muslims may be morally better than many, but their downfall is that they do not recognize the name of Christ; no other name makes salvation possible.
This is the stone which was set at nought of you builders, which is become the head of the corner.