Slow fashion is defined against fast fashion — the model of very short design-to-shelf cycles and low prices that assume short garment lifespans.
In wardrobe terms it means buying less often, keeping things longer, repairing rather than replacing, and treating cost per wear rather than price as the relevant number.
Overlaps with but is not the same as a minimalist wardrobe. Slow fashion is about how you acquire; minimalism is about how much you own. You can own a lot of long-lasting clothes.