Transformational rules are a set of guidelines in linguistics that describe how sentences can be transformed from one structure to another while maintaining their meaning. These rules help explain how different sentence forms, such as questions or passive constructions, can be derived from a basic sentence structure. In the context of Noam Chomsky's theories, transformational rules are essential for understanding the complexities of human language. They illustrate how speakers can manipulate sentence elements, allowing for a rich variety of expressions and communication styles. This concept is fundamental in the study of generative grammar.