Psalms 121

1 Pilgrims to Jerusalem sang this psalm as they walked up Mount Zion towards the Temple.

This first verse might have a couple of meanings. In a battle, one might look to the mountains hoping for reinforcements. On the other hand, one might look to the mountains (or specifically Mount Zion) in prayer to God.

2 Whatever the interpretation of the first verse, the second is very clear that the pilgrim's true hope is in God alone. Since God is powerful enough to create heaven and earth, He would be powerful enough to help in any situation. This is a statement of God's omnipotence.
3 God is omnipresent, which means He can actively watch everything that happens in the universe at all times. He makes sure that when a believer stands on His precepts that they will not fall away.
4 God does not get tired and does not take naps. Instead, He is constantly awake, and nothing can get past Him unnoticed.
5 The believer is comforted knowing that God will keep them safe. Since the believer is exposed to the dangers of this world, we should see this as a promise for spiritual safety rather than a physical one. God certainly will intervene to protect our physical bodies, but we do not see this all the time. We must believe that God either protects us or allows the world to affect us as best fits His purpose for us. What God guarantees us is that our spirits will be delivered safely into His presence on Judgement Day (Psa 121:7).
8 The use of the word "forevermore" indicates the spiritual implications of this passage. The human body will eventually die, but the godly soul is preserved. No evil power can wrestled a saved person from the hand of God in either this lifetime or the next.