By saying that he “will not be shaken,” David was talking about the unique sense of security felt by believers. God does not exempt believers from the day-to-day circumstances of life. Believers and unbelievers alike experience pain, trouble, and failure at times (Matthew 5:45). Unbelievers have a sense of hopelessness about life and confusion over their true purpose on earth. Those who seek God, however, can move ahead, confident in knowing what is right in God’s eyes. They know that God will keep them from straying off his chosen path.
(Source: Life Application Bible)
5 LORD, you have assigned me my portion and my cup; you have made my lot secure.
6 The boundary lines have fallen for me in pleasant places; surely I have a delightful inheritance.
7 I will praise the LORD, who counsels me; even at night my heart instructs me.
8 I have set the LORD always before me. Because he is at my right hand, I will not be shaken.
9 Therefore my heart is glad and my tongue rejoices; my body also will rest secure,
10 because you will not abandon me to the grave, nor will you let your Holy One see decay.
11 You have made known to me the path of life; you will fill me with joy in your presence, with eternal pleasures at your right hand.