It is a fallacy that deaf people are not intelligent. Our director for the deaf, Ian Funnell, is totally deaf but he works as a highly qualified draughtsman in the aerospace industry. He is also able to read most literature, only requiring explanations of new words now and then. Whenever possible, I have to use simpler words for deaf readers. Ian may understand, but many deaf people do not, because of their limited ability to learn words quickly enough.
Concepts usually require manipulation of words and ideas.
If one has a very limited vocabulary, then the ability to understand concepts is very much reduced. It takes great patience. In articles, we have to provide pronunciations for some words. Sometimes this can be hard, because certain sounds are not easy to show in letter form. Of course, in most cases only a few words require such painstaking work, but the above sentence will suffice as an example.