1 Blessed is the man who walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers; 2 but his delight is in the law of the Lord, and on his law he meditates day and night. (ESV)
Those who follow unethical advice, who follow peer pressure to do evil things and nay say that which is righteous shall not be blessed. On the other hand, if you love the law and think upon what is right and wrong you will be blessed.
3 He is like a tree planted by streams of water that yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers. 4The wicked are not so, but are like chaff that the wind drives away. (ESV)
Follow the God's commands and you will be like a tree with strong deep roots. You will get nurisment (planted by streams), will be productive (bare fruit) and full of life (leaves that don't wither). The wicked will not revieve this blessing, but not be steady, not firmly rooted and at the mercy of the winds of change (be like chaff on wheat that is blown away).
5 Therefore the wicked will not stand in the judgment, nor sinners in the congregation of the righteous; 6for the Lord knows the way of the righteous, but the way of the wicked will perish. (ESV)
Wicked will not be found innocent at the time of judgment and will be separated from the righteous. Those who do ill will perish, but the Lord will keep those who good.