To illustrate is to make something more clear or visible. Children's books are illustrated with pictures. An example can illustrate an abstract idea.
The word illustrate comes from the Latin illustrare 'to light up or enlighten.' Pictures in a book enlighten the reader, and a good example can enlighten someone on a complex topic. Another use: The government's actions illustrate how out of touch it is with the people. The history of this word illustrates how language changes over time, and how knowing the original meaning can help you understand the word in a deeper way.