Decreasing evenly means the decs are spread around the round (or spread across the row).
Imagine that you need to make your sock a bit smaller, say 4 stitches smaller. You could do all the decreases next to each other but that would look odd and maybe pucker the fabric.
So what you do in this case (think of the circularity of the sock as like a clock face) is decrease at 12, 3, 6, and 9 o' clock: at four points spaced out evenly. Like the four compass points.
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