tn The short blessing uses the jussive throughout, here the Piel jussive with a pronominal suffix. While the jussive has quite a range of nuances, including wish, desire, prayer, or greeting, the jussives here are stronger. The formal subject of the verb is the Lord, and the speaker pronouncing the blessing is the priest, notably after emerging from the holy of holies where atonement has been made. The Lord says in this passage that when the priest says this, then the Lord will bless them. The jussive then is an oracle, not a wish or a prayer. It is a declaration of what the Lord imparts. It is as binding and sure as a patriarchal blessing which once said officially could not be taken back. The priest here is then pronouncing the word of the Lord, declaring to the congregation the outcome of the atonement.
tn The verb “to keep” concerns the divine protection of the people; its basic meaning is “to exercise great care over,” “to guard,” or “to give attention to” (see TWOT 2:939). No doubt the priestly blessing informed the prayer and promise that makes up Ps 121, for the verb occurs six times in the eight verses. So in addition to the divine provision (“bless” basically means “enrich” in a number of ways) there is the assurance of divine protection.