A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 1:1The character and condition, and the present and future destiny, of the pious and the wicked are described and contrasted, teaching that true piety is the source of ultimate happiness, and sin of misery. As this is a summary of the teachings of the whole book, this Psalm, whether designedly so placed or not, forms a suitable preface. (Psa 1:1-6)
Blessed--literally, "oh, the happiness"--an exclamation of strong emotion, as if resulting from reflecting on the subject. The use of the plural may denote fulness and variety (Ch2 9:7).
counsel . . . way . . . seat--With their corresponding verbs, mark gradations of evil, as acting on the principles, cultivating the society, and permanently conforming to the conduct of the wicked, who are described by three terms, of which the last is indicative of the boldest impiety (compare Psa 26:4-5; Jer 15:17).
psa 1:2law--all of God's word then written, especially the books of Moses (compare Psa 119:1, Psa 119:55, Psa 119:97, &c.).
psa 1:3like a tree-- (Jer 17:7-8).
the rivers--canals for irrigation.
shall prosper--literally, "make prosper," brings to perfection. The basis of this condition and character is given (Psa 32:1).
psa 1:4not so--either as to conduct or happiness.
like the chaff--which, by Eastern modes of winnowing against the wind, was utterly blown away.
psa 1:5stand in the judgment--be acquitted. They shall be driven from among the good (Mat 25:45-46).
psa 1:6knoweth the way--attends to and provides for them (Psa 101:6; Pro 12:10; Hos 13:5).
way of the wicked--All their plans will end in disappointment and ruin (Psa 37:13; Psa 146:8; Pro 4:19).