Picture a ladder of steps with something placed on one of them. Low-grade says the thing sits near the bottom step. The ladder itself changes with the subject — how good a material is, how serious an illness is, how a road rises — but the word does the same job every time: it points at the low end and leaves the middle and the top for other words.
Two branches
等級表上靠底部那幾階
這一支最貼近核心畫面：有一個公認的等級表，這個東西被放在低的那一端。差別只在表在量什麼。
adjective · B2
Not very good in quality, because the material or the work behind it sits near the bottom of the scale. It is used most often about cheap raw materials such as paper, coal, metal or plastic.
低品質的
grade 的另一條線：坡度
這一支的 grade 量的不是好壞，而是地面往上升的幅度；「低」還是指同一端，只是那條刻度被畫在山坡上。
adjective · C1
Used about a road, a path, a railway line or other built ground that rises only a little, so it stays nearly level and is easy to travel along.
坡度平緩的