In the opening lines of this Psalm, chapter 62, we can see the psalmist speaking to his soul, exhorting it to remain in a place of trust, a stable place for it. His trust was in God, and it would not be shaken.
It is clear that we must rest in God, because when we place one foot on the rock and the other on shaky ground, the shaky ground will perish. David makes it clear that every man who places his trust in God will rest and will not stumble or slip.
In verse 6 of this chapter, we are told of the enormous trust that we must place in our God, who is the eternal rock. If we stand firm in Him, we will not fall. This is what David spoke of and ordered his soul to do: to rest in the tranquility that is only found in the Lord.