From the Lojban web site FAQ (http://www.lojban.org/files/brochures/faq.htm):
cmavo: a "little word" showing structure rather than carrying meaning
In fact, of course, most of them do carry meaning (I guess the elidable terminators don't; they are true structure words). Many carry meaning as their sole purpose; for example, numbers, lerfu, and everything in selma'o UI. But it is true that the cmavo as a class determine the syntax of sentences. mi'e jezrax