The person who meditates (studies) the scriptures, is similar to the tree that is planted (not by himself) by others (instruction), will be productive (spiritual productivity), and prosper and blossom, and grow into a full and fulfilling life.
Your life is in your soul, and what is in your soul is your life.
Streams of water refer to Bible doctrine, that flows along and near the tree, from which the tree reaches out with its roots and absorbs into itself, thus growing richly and healthy.
The tree produces fruit of many different types (different trees have different production).
The tree produces, because of the constant flow of water (doctrine) and rich soil (foundation of doctrine), all from the source of God (truth) and not of this world (ungodliness, sinfulness, scornfulness).
Whatever the tree does, through doctrine, it prospers. Therefore, apply this analogy to your own life.
You have a big world out there from which you can draw your livelihood. You learn academically, and through experience, but if it all comes from worldliness (not of God), then all you have comes to nothing.
However, if you draw from doctrine, and allow yourself to be planted (taught) from the right sources, if you learn doctrine, thus filling up your soul with truth, and allow it to flow through your mind (your thought process), then the resultant spiritual growth will never fail, and you will never fail in your spiritual life.
This does not mean that you will succeed in this temporal world. Your life is destined for bigger things in eternity in heaven with God. But if you fail in your daily Bible studies, then you will fall short in eternity.
The tree takes in water constantly and daily, for all of its life. It is nourished constantly and daily throughout its life. So too, you need to be nourished daily throughout your life, in order to be spiritually successful.
To ignore doctrine, is to refuse water. And to refuse water leads to poor health or death. Therefore, desire doctrine, pursue doctrine, and acquire the success and happiness and fulfillment that everyone strives for in life, but never find from this world.
The completion of your life is located in Bible doctrine. You just have to reach out for the water (doctrine) and drink it (learn it).