Small common words are those like the, a, with, of, by, and from.
Most non-Scientologists just assume that a normal adult, with English as his/her native language, understands them fully as a matter of course. When Bill Clinton was under fire for dallying with a young intern in the White House, the country generally laughed at his comment that it all depends on the meaning of "is".
In fact, these small words are very difficult for most people to define accurately. There is a Scientology course called the Key to Life Course, and part of this involves extensive word clearing on about sixty of these small common words.