The sounds of words do not normally contain any hint of their meaning. So signs must not look like their meanings. The arbitrary symbols must be used in strict order. In fact, chimps will have to come up with new, creative combinations to describe new, previously unencountered situations. So if Bill Gates were to introduce trains to get from one room to another in his house, we wouldn't think twice before calling it a 'housetrain', even though we have never heard the word before.
Chimps have to be able to do the same.
The symbols will have to be relative and not absolute. Virtually no word in any human language has only one meaning. Rather, words have meanings only in context. Take a look at these examples. The table was covered with junk food.