The Destruction of the Unrighteous (Psalms 1:4-5).
‘Not so the wicked!
They are like the chaff which the wind blows away.
Therefore the wicked will not stand in the judgment,
Nor sinners in the assembly of the righteous.'
The opposite is true of the wicked. They are not fruitful. They are not firmly grounded and planted. They are not good grain. They are rather chaff, the outer husk, the useless and lifeless part of the grain. They have no substance, they have no value, and instead of being rooted in the ground they are eventually blown away by a puff of wind as useless and worthless. They cannot produce fruit. They are chaff.
So just as the chaff is blown away when the grain is tossed up, separated from the grain by the wind, so are the careless and sinful blown away in their frailty. They are blown away when God's wind blows on them. This picture of sinners as chaff is a constant one in the Old Testament (Psalms 35:5; Job 21:18; Isaiah 29:5; Hosea 13:3), and in the New (compare Matthew 3:12; Luke 3:17), and the wind is compared in one place with ‘the Angel of YHWH' (Psalms 35:5), that mysterious figure Who is the representation of God Himself. It is God Who blows them away.
When judgment comes they will not be able to stand (Psalms 5:5; Psalms 130:3), they will have no place in the gathering of the righteous. The thought is not specifically of some final Judgment Day, but of whenever God's judgment comes on them (for an extreme example see Numbers 16). It is a principle of Scripture that God continually judges the wicked, even before the day of His final judgment which finally completes that judgment. Because sin must be judged and must be condemned God deals with it continually in all kinds of ways. And in the face of that judgment the wicked will be blown away. They will not be able to prevent it. They will be unable to stand. If you ‘stand in the way of sinners', you will not be able to stand at the judgment.
‘The assembly of the righteous.' Israel were known as ‘the congregation, the assembly' which represented the whole of Israel as they gathered together as God's people. But here already we see the idea of the remnant within Israel (Isaiah 6:13), the true Israel (Isaiah 49:3 with 5), the assembly of the righteous. For not all of Israel were Israel. Not all were faithful to God and the covenant. And that separation will become apparent by judgment, when the righteous are gathered as one, separated from the wicked (Matthew 25:31-46; Matthew 13:30; Matthew 24:31).