A cwm is a steep-walled semicircular basin or valley on the side of a mountain. Originally formed by a small (comparatively speaking) glacier, one which fed into a larger glacier valley. Pronounced 'koom'. These are more often called cirques, and much more information can be found in that node.
I node this here this 'cus cwm is one of the two (or five) words I know that uses 'w' as a vowel*. It is actually a Welsh word for valley, but Webster considers it a valid English word.
*The others are crwth (a crwth is the same thing as a crowd, an ancient Celtic stringed instrument), along with the much rarer gweduc, cwtch, and bwbachod, which are rarely, if ever, seen in English writings.