This psalm serves as an introduction, or prologue, to the collection of the Psalms in its final form. It is a teaching, or wisdom, psalm advising the readers to dedicate themselves to the study of the Law of God, the Torah, and warning them of the consequences if they don’t.
The psalm consists of two strophes. The first one (verses 1–3) describes the truly pious person, stating what he refuses to do (verse 1), what he does (verse 2), and then describing him as like a healthy tree (verse 3). The second strophe (verses 4–6) compares the evil person to chaff, which is blown away by the wind. Such a person will not share the future happiness of the righteous but will instead be destined to destruction.
Bratcher, Robert G., and William David Reyburn. A Translator’s Handbook on the Book of Psalms. New York: United Bible Societies, 1991. Print. UBS Handbook Series.