Microsoft LUIS (Language Understanding Intelligent Service) is a cloud-based service that helps developers build applications capable of understanding natural language. It allows users to create custom models that can interpret user intents and extract relevant information from text, making it easier for applications to respond appropriately to user queries. LUIS uses machine learning to improve its understanding over time, adapting to the specific language and phrases used by different users. It can be integrated into various applications, enhancing user interaction through chatbots, voice assistants, and other natural language processing tools.