Something that's interdisciplinary covers more than one field of study. If you take an interdisciplinary science and literature class, you might read a science fiction novel and then explore the scientific ideas behind it.
The word interdisciplinary can be broken into its parts: inter-, which means "between" in Latin, and disciplinary, which is from the Latin disciplina and means teaching or knowledge. Interdisciplinary means between fields, but they don't have to be unrelated disciplines. You can do interdisciplinary work between two sciences, for example. An interdisciplinary conference on biology and computer science might include workshops on both fields.