This psalm is chosen by the early editors of the book of psalms to set the tone for the whole.  The psalm draws a sharp contrast between the "righteous" and the "wicked."  The righteous are "like a tree planted by rivers of water," while the wicked are "like chaff the wind drives away."  Chaff is the part of wheat which is separated from the grain on the threshing floor; dried up, dead, and useless.  The pslamist is drawing a stark contrast between life and death in the spiritual sense.

Many scholars suggest that Psalm 2 was originally joined to Psalm 1.  You may hev noticed that there is no title given for Psalm 2, suggesting that it is to be read together with the preceding chapter.

Additionally, the two psalms share a common picture, though it is obscured by most English translations.  The Hebrew word for "meditate" in 1:2 is the same as that for "plot" in 2:1.  The word means "to murmur under one's breath" and draws a picture of fervently whispered prayers or conspiratorial words uttered behind the hand.

Together, then, Psalms 1 and 2 contrast the friends of God who meditate on the word with the enemies of God who plot their own schemes.