My soul finds rest in God alone;
my salvation comes from him.
There is a huge, massive truth in this verse. It is a truth about the very nature of man. It is a truth about the key to a happy, fulfilled existence.
In this passage, the word soul is the hebrew word nephesh, which is not referring to the immortal soul, it is referring to "...self, life, creature, person, appetite, mind, living being, desire, emotion, or passion..." This part of the man will find peace in God alone.
As humans though we seek to find peace in everything imaginable, in every created thing. Peace and happiness will elude us as long as we seek it in these temporary things. True peace will only be found in the creator. The only one that is worthy of our passions, appetites, thoughts, emotions, etc. is God Himself!
God knows this, so he has told us this time and time again. He commands it in the ten commandments, "...You shall have no other gods before me." Jesus tells us that the summary of the Law and the Prophets (the Old Testament) is to "...love the Lord your God with all your heart, all your soul, all your mind, and all your strength." These commands were not meant to control and bind us, but to liberate us to a free and happy existence.
Lay aside all of those other gods that you are seeking after, and look to the only one that can give your soul peace. Your salvation from those other things is in Him.