Nobody knows exactly what crop or grain is represented by these words. That Joseph Smith would not be able to describe in 1829 English every word or term used by the Nephites is evidence for the authenticity of the Book of Mormon:
"Together with 'neas and sheum' of Mosiah 9:9 and 'cureloms and cumoms' of Ether 9:19, we have some very convincing examples of what are technically known as hapax legomena. Linguistically, such terms are a part of almost all ancient records. Indeed they become a check on their age. Hapax legomena are terms which cannot be translated, only transliterated-that is, put into the sounds of a language." (Richard H. Cracoft, Neal E. Lambert, A Believing People: Literature of the Latter-Day Saints, p. 143)