A fight with your brother over who ate the most food may be illustrative of your competitive relationship with him. Something illustrative means it is a telling example of something else.
Within this word, you see illustrate which means to make something clear by using pictures or examples. Something is illustrative when it paints a perfect picture of a subject. If you're trying to describe something, an illustrative example might make it easier for your audience to understand what you mean. The tidiness of your sock drawer might be illustrative of your compulsive (or messy) nature.