When explaining complicated information to clients about a product or service how do you make sure that they understand what you tell them?

Ask them to explain what they want out of the product or service this will help set your complexity level of their understanding now listening is another thing.

Have at hand several different ways of explaining the same information: examples, metaphors, analogies, stories, charts, testimonials, quotes, drawings, diagrams, user demonstrations. Explain your complicated information using one method, then ask questions to check for understanding. If the client doesn't yet grasp the concept, try a second method. See http://www.turningpointpresentations.com/index.php?pr=Newsletter_3.