Rasa NLU is an open-source natural language understanding framework designed to help developers build conversational AI applications. It enables machines to understand user inputs by extracting intents and entities from text, allowing for more effective communication between users and chatbots or virtual assistants. The framework is part of the larger Rasa ecosystem, which includes tools for dialogue management and deployment. Rasa NLU supports multiple languages and can be customized to fit specific use cases, making it a popular choice for businesses looking to create tailored conversational experiences.