What is the idea-expression dichotomy?

One of the fundamental properties of copyright law is that only expressions are protected, not ideas. For example, while the plot, dialogue, images, etc. of The Matrix movies are protected by copyright (because these are all expressions of an idea), the idea of a war between computers and humans, or a virtual reality world being used as a prison, is not protectable. This property of copyright law, and the distinction between ideas and expressions, is often referred to as the idea-expression dichotomy.