The book of Psalms is a series of songs Jewish people sang in their synagogues. They were written over many years and by a variety of writers. Since they have been translated into English, they lose their poetic sound, but they still contain many word pictures, just as poetry often does. This first Psalm is a good example. We are told how a righteous person differs from a wicked one. Righteous people are like trees, and wicked people are like chaff. Righteous people are shown as stable and productive, wicked people as passing and useless.
Pictures like this are very powerful. When making a decision to do the right or wrong thing, you are deciding between being a tree or chaff. It’s more than deciding on a behavior; it’s deciding the kind of person you are.