cwt is the written short form of hundredweight, an old unit of weight. You mainly see it on labels, shipping papers, and older British writing, not in ordinary talk. A hundredweight is not a neat “100 of something” in every country. In Britain the long hundredweight is 112 pounds (so that 20 of them make a British ton of 2240 pounds). In the United States the short hundredweight is 100 pounds. So the same letters can mean two different amounts. The word is technical and a little old-fashioned. Students rarely need it unless they read trade documents or old measures.