There is a big chance that chapter 91 is a continuation of chapter 90. So, to understand the first properly, we need to read both consecutively.
BOOK IV – NIV
A prayer of Moses the man of God.
1Lord, you have been our dwelling place
throughout all generations.
2Before the mountains were born
or you brought forth the whole world,
from everlasting to everlasting you are God.
3You turn people back to dust,
saying, “Return to dust, you mortals.”
4A thousand years in your sight
are like a day that has just gone by,
or like a watch in the night.
5Yet you sweep people away in the sleep of death—
they are like the new grass of the morning:
6In the morning it springs up new,
but by evening it is dry and withered.
7We are consumed by your anger
and terrified by your indignation.
8You have set our iniquities before you,
our secret sins in the light of your presence.
9All our days pass away under your wrath;
we finish our years with a moan.
10Our days may come to seventy years,
or eighty, if our strength endures;
yet the best of them are but trouble and sorrow,
for they quickly pass, and we fly away.
11If only we knew the power of your anger!
Your wrath is as great as the fear that is your due.
12Teach us to number our days,
that we may gain a heart of wisdom.
13Relent, Lord! How long will it be?
Have compassion on your servants.
14Satisfy us in the morning with your unfailing love,
that we may sing for joy and be glad all our days.
15Make us glad for as many days as you have afflicted us,
for as many years as we have seen trouble.
16May your deeds be shown to your servants,
your splendour to their children.
17May the favour of the Lord our God rest on us;
establish the work of our hands for us—
yes, establish the work of our hands.
a 17 Or beauty
1Whoever dwells in the shelter of the Most High
will rest in the shadow of the Almighty.
2 I will say of the Lord, “He is my refuge and my fortress,
my God, in whom I trust.”
3 Surely he will save you
from the fowler’s snare
and from the deadly pestilence.
4He will cover you with his feathers,
and under his wings you will find refuge;
his faithfulness will be your shield and rampart.
5You will not fear the terror of night,
nor the arrow that flies by day,
6nor the pestilence that stalks in the darkness,
nor the plague that destroys at midday.
7A thousand may fall at your side,
ten thousand at your right hand,
but it will not come near you.
8You will only observe with your eyes
and see the punishment of the wicked.
9If you say, “The Lord is my refuge,”
and you make the Most High your dwelling,
10no harm will overtake you,
no disaster will come near your tent.
11For he will command his angels concerning you
to guard you in all your ways;
12they will lift you up in their hands,
so that you will not strike your foot against a stone.
13You will tread on the lion and the cobra;
you will trample the great lion and the serpent.
14“Because he loves me,” says the Lord, “I will rescue him;
I will protect him, for he acknowledges my name.
15He will call on me, and I will answer him;
I will be with him in trouble,
I will deliver him and honor him.
16With long life I will satisfy him
and show him my salvation.”
Psalm 91: 12 is a verse that was quoted a lot, since Jesus used it against the devil, during the temptation. Due to this, it may occur that King David was addressing the Son of God in this chapter. Another reason to believe this is that David was addressing God in chapter 90.
However, some evidence from chapter 91 itself shows that the pronoun ‘you’ cannot mean the Son of God. The person, David was addressing, is a vulnerable person. He’s someone who needs God’s protection very much. He’s someone with limited spiritual powers. David maybe referring to himself or to whoever believes in God and loves him.
Verse 12. To carry them in their hands is a metaphor, and signifies a perfect execution of their custody, to have a special care of them, and therefore is rather expressed so, than carrying them on their shoulders. That which one carries on their hand they are sure to keep. The Spaniards have a proverb when they would signify eminent favour and friendship, `they carry him upon the palms of their hands,’ that is, they exceedingly love him, and diligently keep him. Lest thou dash thy foot against a stone. He persists in the metaphor: children often stumble and fall unless they are led and carried in hands and arms. By stones are meant all difficulties, objections, perils, both to the outward and inward man, as Christ is said to take care of hairs and sparrows, that is, of everything even to a hair. Now we know what this charge is, saving that Zanchy adds also the metaphor of schoolmasters, and says that we are poor rustic people, strangers; but being adopted into the household of God, he gives his most noble ministers, the angels, charge, first of our nursing and then of our education; when we are weaned, to instruct us, to admonish, to institute, to correct us, to comfort us, to defend us, to preserve us from all evil, and to provoke us to all good. And these angels, seeing we are so dear to God, that for our sakes he spared not his own Son, take this charge with all their hearts upon them, and omit nothing of their duty from our birth to the end of our life. Henry Lawrence, in “A Treatise of our Communion and Warre with Angells,” 1646.