The image of keys is already found in Isaiah 22:22 (see also the image of the opened and shut doors in Revelation 3:7). The act of handing over the keys signifies conveying power. The one who then holds the keys has the same power as the lord of the house. The power of the keys of the kingdom pertains to binding and loosing on earth which will be affirmed in heaven: that is, by God himself.1
19 Je te donnerai les clés du royaume des cieux: ce que tu lieras sur la terre sera lié dans les cieux, et ce que tu délieras sur la terre sera délié dans les cieux.