The Microsoft Azure Luis Authoring Client is a tool used for building and managing Language Understanding (LUIS) applications. LUIS is a cloud-based service that allows developers to create natural language understanding models to interpret user input and extract relevant information. With the Authoring Client, users can easily define intents and entities, train models, and test how well their application understands and processes different types of input. The tool provides a user-friendly interface for designing and fine-tuning language understanding models, making it easier for developers to create powerful and accurate conversational interfaces for their applications.