They are like trees planted by the streams of water; which yield their fruit in its season and their leaves do not wither. In all that they do, they prosperPsalm 1:3
The image of a tree in this verse applies to a godly person. The verse continues by describing what the life of a godly person is like. The psalm, therefore, flows from what a godly person does not do (verse 1), then moves to what a godly person does (verse 2), and now what the life of a godly person is like. The context of this image is the Middle East region where vegetation has to be planted along streams or other natural sources of water for them to survive. Sometimes vegetation was planted along the canals because in that dry region the water table was too long for tree roots to reach. Those living in different contexts where the water table is just high would not understand this imagery.
An orchard grower would not plant fruit trees in the middle of the desert where the water table is very low. They would eventually wilt. For a fruit tree to grow healthy and bear fruits, it must enjoy an abundant supply of water. Water in this poetic narrative refers to the word of God. Therefore, a godly person is like someone planted in the word of God. Evergreen leaves are a symbol of success while wilting is a symbol of failures; therefore the psalmist is simply saying success in life comes with relying upon the word of God. Finally, though a tree is planted in the middle of the stream with abundant water, the bearing of fruits happens in due season. Therefore, success happens at God´s appointed time. Our duty is to soak ourselves in the waters of the Word of God and He takes care of the success at His appointed time.