Imagine a drink so potent, so intense, it's served with absolutely nothing to dilute its power. That's the imagery John uses here for God's wrath.
The verse speaks of God's wrath being "poured out without mixture." In ancient times, wine was almost always mixed with water. Serving wine unmixed was reserved for the most potent, often medicinal or ceremonial, purposes. Here, it signifies that God's wrath is not tempered or softened in any way. There's no grace, no dilution, no mercy offered in this final judgment. It is pure, unadulterated retribution for sin.
This imagery highlights the severity and finality of God's judgment. It's a stark contrast to the grace offered during the time of the "everlasting gospel" (v. 6), emphasizing the critical importance of responding to God's mercy while it is still available.