In the early 1900s, a horse by the name of Clever Hans appeared to be able to do math. They asked the horse something like, "What's 3 = 7," and he tapped his hoof ten times. At the time, it seemed like the horse could undersand questions and perform calculations. But people later discovered that Hans wasn't do any of those things. He was merely reading the body language of the people around him. Whenever Hans reached the correct number of taps, he observed a change in people's body language--and that prompted Hans to stop tapping. In other words, people unknowingly sent him information through their body language--and Hans was able to pick up on it.
"The Clever Hans Effect" refers to instances of someone unkowingly giving someone else information through body language, expressions, clothing, etc., and after doing so, the second person presents that information to the first person. In some cases, the second person is not even aware of how he's receiving that information. For instance, he might not realize that he's learning about someone through her body language or clothing.