The expression “kingdom of heaven” is equivalent to the expression “kingdom of God”, which is used in the parallel account in Luke (cf. Luke 6:20). The word “heaven” in the expression “kingdom of heaven” does not refer to the location of the kingdom, but the authority of the kingdom.
The expression “kingdom of heaven” is only found in the Matthew. The evangelist uses this expression instead of the expression “kingdom of God” found in the other gospels (compare Mark 4:26 and Matt 13:24). It has been thought that Matthew, writing primarily for a Jewish audience, has adopted the Jewish practice of periphrasis to avoid referring directly to God out of reverence for his name. The other gospels, who wrote for Gentile or mixed audiences, do not use this periphrasis.