The Book of Psalms is the Bible's songbook: 150 prayers, poems, and songs written over hundreds of years and gathered into one volume. Some are joyful, some are desperate, some are raw with grief or anger. They cover the full range of human experience addressed directly to God. Psalm 1 is their prologue, placed before all 150 as a deliberate introduction to the Psalter as a whole.
Psalm 1 introduces a contrast that runs through the entire Psalter: the life oriented by God versus the life that isn't. The person who meditates on God's word has roots, direction, and purpose, compared to a tree planted by a river, stable and fruit-bearing regardless of season. The latter person is compared to chaff, the husk blown off grain at the threshing floor. He is not described as wicked or evil, but simply as someone without direction, without divine purpose to tether his existence to anything more meaningful than his own immediate impulses.