Grading

Grading a seam means trimming each layer of a multi-layer seam allowance to a slightly different width instead of cutting them all even - typically the layer closest to the body trimmed narrowest, each one after it left a hair wider. Stacked seam allowances of identical width create one thick, sharp-edged ridge under the fabric; staggering the widths spreads that bulk out so it tapers instead of stopping abruptly, and the finished seam presses flatter and shows less from the outside.

It matters most anywhere several layers meet at once - a collar or cuff with interfacing plus two fabric layers, a waistband, a curved facing - and it's the detail I mentioned in Collar as the difference between a clean finish and an amateur one. Skip it on a straight, single-layer seam and you likely won't notice; skip it on a collar point or a curved facing and the ridge shows right through, especially in lighter fabrics.

Worth keeping separate in your head from "pattern grading" - the unrelated, if similarly named, process of scaling a whole pattern up or down across a size range (the innovation behind the Butterick pattern company, see Notable Figures). Same word, two different jobs: one trims fabric after cutting, the other resizes the pattern before you ever cut.