English Basic Explanation - Chapter 127
A number of commentators explain this Psalm as a poem David composed upon being informed that his son Shelomo, rather than he, would be given the privilege of building the Bet Ha'mikdash. In the first two verses, David declares that it is God, not man, who determines when a building will arise. Regardless of how hard the laborers toil or how carefully the watchmen guard the building site, the project will materialize only with the direct assistance and support of the Almighty. David here accepts God's decree forbidding him from building the Mikdash, recognizing that if God does not wish for him to build it then even if he would try the endeavor would assuredly fail.
In the Psalm's final verses (3-5), David turns his attention to the great blessing of children, whom he compares to "arrows in the hands of the mighty warrior" (verse 4). A warrior's most valuable asset is effective weaponry; for a religiously conscientious Jew, the greatest commodity and blessing is children who follow the Torah traditions that he received from his parents. David thus expresses his gratitude for the fact that his son would perpetuate his legacy and heritage. Although he desired to personally oversee the building of the Mikdash, he found solace in the fact that he leaves behind a son faithful to his teachings, and who will fulfill Am Yisrael's collective dream of having in their midst an abode for the Shechina (Divine Presence).