The word congruent in regard to a verbal illustration means harmonious or consistent. Therefore a "congruent illustration" would be an example or demonstration that is appropriate to the content that one is trying to convey.
Illustrations are typically used to elaborate on abstract and complex concepts. If a teacher is trying to explain something complicated, she may want to connect the concept to something that her students already know. A good illustration builds upon the audience's background knowledge. A lawyer may call upon the jury's past experiences to help them understand the situation under their judgment. A politician compares the current economic situation to a similar experience in the past ensure his audience how the situation has been dealt with previously. A doctor may use an illustration to demonstrate how the patient's heart works.
Verbal illustrations include similes, metaphors, and allegories. While similes and metaphors are generally used in artistic comparisons, an allegory is a type of verbal illustration that is used to explain a particularly difficult concept. The allegory is a long type of illustration, usually a paragraph or more in length. In teaching and rhetoric, the allegory is considered a powerful tool for illustrating one's point of view or explaining a complex concept.
Illustrations need to be clear, accurate and relevant to the topic. An appropriate or congruent illustration is therefore more effective than an incongruous or inappropriate one. A good illustration should be easy to understand and require little explanation. The author or speaker should limit her use of illustrations. Too many illustrations in one lesson or argument will overwhelm the audience. The speaker or author must use only illustrations that connect with the experiences and knowledge of her audience. Illustrations should only be used to elaborate on the content at hand, and should not be used simply for their own sake.