cu is the written short form of cubic. You use it for volume — how much space is inside something — which you get by multiplying length by width by height. 3 cu m means three cubic metres; cu ft means cubic feet; cu in means cubic inches. You meet this on packs, in older books, and with British or American measures. In a science class today, people more often write a small 3 in the air, like m³ or cm³. When this cu means cubic, you read it aloud as "cubic", not as the letters C-U.